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500 years of NOT teaching THE CUBIC FORMULA. What is it they think you can’t handle?


Welcome to another an Mathologer video. If I
were to wake you up in the middle of the night, I’m sure that most of you would be
able to rattle off the quadratic formula: minus B plus or minus the square root of B
squared, and so on. But along with all that quadratic fun
you had in school, most of you would have tortured a few carefully cooked up cubic
equations and maybe even a quartic or two. Right? But did you ever wonder why no
one ever taught you the cubic formula or the quartic formula? Did your teachers ever
even mention the higher degree counterparts of the incredibly useful
quadratic formula? Probably not. Why is that? Is it a cultish
mathematical conspiracy? Are they all hiding some deep dark secret? Well
today’s Mathologer is your invitation into the hidden polynomial brotherhood.
Okay, here’s the general cubic equation which we want to solve. And here’s its
solution, the cubic formula in all its glory. Whoa that looks … interesting. So maybe
that’s our answer right there. That’s way too complicated to memorize and use. A
pretty good reason not to teach it, right? Wrong! Sure the full cubic formula
doesn’t exactly roll off the tongue. But it’s not that hard to make sense of. For
starters the two monsters inside the cube roots actually only differ in a
minus sign. But we can also simplify things dramatically by some
straightforward pre-processing. The general cubic equation can be reduced to
solving a much simpler cubic with a genuinely simple cubic formula. What do I
mean by pre-processing? Well, first, if you divide by the leading coefficient a that
gives an equation with the same solutions but with a leading coefficient
of 1. So we may as well turn all the a’s into 1s. That already looks a lot
simpler. The next most frequent coefficient in the formula is b. Now it’d
be good to get rid of all those b’s and it turns out we can. There’s a second
easy pre-processing step which I’ll motivated soon. This pre-processing
allows us to assume b equals zero and so we just have to worry
about solving a simple cubic without the green quadratic term. Pretty simple, right?
Maybe not kindergarten simple but definitely not a big leap from the
quadratic formula. Again, by performing some simple pre-processing, we can reduce
any cubic equation whatsoever to solving one of these simple cubic equations. In
the literature the letters c and d are usually replaced by the letters p and q
and when people say cubic formula they usually have this exact formula in mind. Really very pretty, isn’t it? Okay, let’s
do a quick example to see how it all works.
Let’s choose, well let’s see p equal to – 15 and q equal to – 126.
Sub these numbers everywhere and we get this. The number under the square root
signs pans out to be 3844 which happens to be equal to 62 squared.
Lucky ! 🙂 Now 63-62 that’s 1 and 63 + 62 that’s 125. Cube root of 1 is 1 and cube
root of 125 is 5, and so we finally get mm-hmm so 6 is solution of our cubic and
you can easily check that. Great! Although shouldn’t there be three
solutions? What happened to the others? Don’t worry, we’ll get to them later.
Anyway, all this looks pretty straightforward and very useful for
dealing with cubic conundrums. Also, for people interested in how our mathematics
came to be the discovery of the cubic formula is considered to be one of the
major milestones in the history of mathematics. So, once again, why doesn’t
anybody teach the cubic formula. There are a couple of answers to this puzzling
question, one more surprising than the next. Promise 🙂 Of course I’ll explain it
all in the following. As you’ve probably already guessed this is another master
class Mathologer video and as usual with these long videos we’ll start really
easy and slowly build up to the challenging finale. It’s all broken up
into seven chapters, with truckloads of really interesting cubic maths ahead.
Plus at the very end I’ll throw in solving the quartic equation for free.
Mathematical seatbelts on? Okay here we go. The cubic formula was discovered
independently by two Italian mathematicians: Nicolo Tartaglia and Scipione del Ferro. And then neither del Ferro nor Tartaglia told anyone about their
amazing discovery. Huh? Why not? Well, just like today, the success
of a sixteenth century mathematician depended on the sorts of problems they
could solve. But back then there was no such thing as Publish or Perish.
Instead mathematicians competed for celebrity in mathematical duels. Mathematicians would publicly challenge
others with take-home exams. They would post sets of problems and whoever could
solve more problems was declared to be the better mathematician. And so if you
were the only mathematician with a secret weapon like the cubic formula
then you would have been pretty much invincible. Don’t tell anybody.
Makes sense, right? And also an impressive start to 500 years of NOT teaching the
cubic formula. Anyway after winning a competition featuring nasty cubics it
became clear that Tartaglia had the secret weapon that he knew the cubic
formula. Then the great Gerolamo Cardano started going after him for this formula.
Cardano was a fascinating character. He was a polymath with his fingers in everything:
he was a famous doctor, a top mathematician, a successful gambler, and
so on. Anyway, eventually Tartaglia gave in. Interesting tidbit: Tartaglia gave
Cardano his formula dressed up as a poem. Quando chel cubo con le cose appresso. Se agguaglia a qualche numero discreto Truouan dui altri differenti in esso. I
put a link to the translation and discussion in the video description.
Ok Tartaglia also made Cardano’s swear a solemn oath not to tell anybody else.
Which of course Cardano broke. After learning the formula from Tartaglia
Cardano kept digging and eventually also found the cubic formula in a
notebook of del Ferro that other discoverer. It became apparent that del
Ferro discovered the formula before Tartaglia. In Cardano’s mind this meant
he was now off the hook. He could now tell people about del Ferro’s solution
and he actually subsequently published a book about it. In a way Cardano’s book
was like a Renaissance WikiLeaks and similar to today and not surprisingly
Tartaglia was incredibly annoyed by this wiki leaking and all hell broke loose.
I’ll link to some sources that have the complete story which, apart from all the
maths also feature a murder, a beheading, and a cameo appearance of the
Inquisition. An absolute must-read for all you maths history buffs. Some great
history, right? Now I’d like you to imagine that we’ve all been transported
back to the time of Cardano and his friends. Try not to be distracted by all
the beheading and inquisitioning. And armed with our knowledge of modern high
school mathswe want to challenge one of these big shots to a mathematical duel.
But since they did not tell us about the cubic formula in school we first have to
rediscover it for ourselves. Time for a bit of a revision
of quadratics to prepare ourselves for the cubic monsters to come. How do you
find a solution to this quadratic equation. Sure, that’s a no-brainer, just
plug the coefficients 2, 8 and -6 into the quadratic formula. Another way of
finding the solutions taught in school since time immemorial is “completing the
square”. This actually translates into the way of
deriving the quadratic formula from scratch. Let me show you a Mathologerized
version of completing the square for this particular quadratic. Ok, start by
dividing through by the leading coefficient. Alright getting there.
Next, interpret x squared as the area of an actual physical square with side
lengths x. Similarly, we’ll think of 4x as the area of a rectangle with sides x and 4.
Chop the rectangle in two like so and rearranged like this. Adding a little
square in the top right corner completes the left side into a new larger square.
But of course to keep our equality we also have to add the little square to
the right side like that. The left side is not just square-shaped it’s also a
perfect square namely x + 2 all squared. It’s now easy to solve the new
equation. Right? Piece of cake. Of course, there’s also that second minus
root 7 solution which we get if we’re not dealing with physical squares. Anyway
here’s an animated derivation of the quadratic formula for the general
quadratic equation by completing the square.
Enjoy the animated algebra and the music. So we know how to complete the square
geometrically in terms of real squares. This is actually similar to how Cardano
and his friends, and probably also the Babylonians thousands of years ago, would
have thought about the process. Algebraically completing the square
boils down to starting with this other super famous identity here, hammering the
quadratic, to look like the right hand side and then using the squareness of
the left hand side to solve the equation. You’ve all done some version of this a
million times, right? To continue our preparation the
cubic puzzling to come we need a second visual derivation of the quadratic
formula. This very nice derivation also illustrates the geometric meaning of the
quadratic formula. Ready? Okay, to be able to quickly refer back to it, let’s write
the quadratic formula in this form and tuck it away up there. Start again with
the pre-processed form of our equation down there. Then the quadratic
corresponds to a parabola like this. Here’s a nice idea. The parabola has a
special point that turning point down there. Let’s shift the parabola
horizontally until the turning point rests on the y-axis. Shift, shift, shift.
The two blue points, the x-intercepts, are still exactly the same distance apart.
However, the quadratic for this new parabola is now in this simpler form,
super simpler form. Right? It’s just the archetypal y=x^2
parabola just shifted up or down. it’s easy to solve this equation which
amounts to locate in the blue intercepts and now to find the solutions we are
really interested in we simply have to undo the horizontal shift we executed
before and keep track of where the blue points go, there great,
Ok now let’s see this scheme in practice First, how far do we have to shift? Well, that amounts to figuring out the x- coordinate of the green turning point
right? There that one. This would normally be done in school with a little algebra
but we’re on a mission here. So let’s take the express lane and use a little
easy calculus to make short work of this job. If you haven’t seen calculus yet,
doesn’t matter, just run with it or substitute the school algebra. OKay, so
form the derivative of the quadratic, set this derivative equal to zero and solve
for x. There, solve, solve, solve. That’s the x-
coordinate of the turning point and more than that: it’s also the familiar – b/2a, the first orange term in our quadratic formula. So that’s just the shift,
right? Now we can shift our parabola. To obtain
the quadratic equation of the new shifted parabola we replace every x by
x-b/2a. Maybe ponder that for a second. All good? Great!
Now use algebra autopilot to expand and collect like terms. Neat. Of course you
also recognize the fraction the messy constant term, right? There, it’s in the
green bit. Now solve to find the shifted blue points there and finally undo the
shift. Tada! Also very nice, isn’t it? Okay having reexpertized ourselves in
quadratics, we are ready to confront our cubic nightmare. A very natural idea is
to try and mimic completing the square to somehow complete the cube using this
perfect cube formula. Hmm, well the way things line up sort of feels it might
work, right? I think most people would play around with this for a while before
trying anything else. Okay so we really go for it for a day, trying to make
completing the cube pan out. And we come up with … zilch! It turns out it’s not that
easy and maybe it’s time to try something else.
So let’s look to mimic our parabola shifting. Okay, start the pre-processing by
dividing through by the leading coefficient. Graphically this kind of cubic will look
roughly like one of these curves. A cubic equation always has at least one
solution but may also have two or three. As well, every cubic has one special
point, the inflection point. This is the point where the cubic switches from
curving one way to the other. Finally, and this may come as a surprise to many of
you, a cubic graph always features a half-turn symmetry about this special
inflection point. Now let’s shift the inflection point to
the y-axis. For that we need the x-coordinate of this point. I’ll again use
some baby calculus. The inflection point here is characterized by the second
derivative being zero. So differentiate once. Okay. Twice. Set equal to zero and
solve. Solve, solve, solve, solved! That’s it, that’s the x-coordinate. This fraction
-b/3a actually appears in the original monster cubic formula I showed
you. Let me show it to you. There that’s the shift again. Now replace x by x – b/3a
and let the algebra auto-pilot perform our shift. That finishes the
pre-processing. The new coefficients are a messy combo of the original a, b, c, and d
but the fundamental form is one of those simple cubics I mentioned in the intro. So if we can figure out how to solve
this special simple cubic equation, then we can undo the shift to solve the
original equation. All very natural and very pretty.
I just love this stuff. Now to analyze our special cubic, what do p and q stand
for in the picture. Well, q is just the y-intercept.
And p is the slope of the curve at the inflection point. So p is the slope of
this green line. Of course, changing q just shifts the cubic up or down. What if
you vary p? Well here’s an animation of how the shape of the curve changes. So
what we’ve got here is that for negative p we’ve got this rollercoaster
shaped curve and for positive p it’s kind of stretched out like that, all
right? So, again, going into the negative – rollercoaster.
In a little while it will be important to know how many solutions our equation
has. Again, for non negative p the graph is stretched out like this and there can
only be one solution. But for negative p we get a rollercoaster graph like this
and 1, 2 or 3 solutions are possible. To figure out how many
solutions we get for negative p, we need to determine the difference in height
between the inflection point and the two extrema. Okay, because of the half-turn
symmetry, the two green height differences will be equal. Now, how many
solutions will we have? Okay, you got it? Well, if the yellow segment is longer
than the green, as pictured here, then there will be just one solution. It’s
also straightforward to check algebraically when this happens. How? Well
the extrema of the cubic occur when the derivative is zero. And the derivative of
the cubic is a quadratic and so we just have to solve a quadratic equation to
pinpoint the coordinates of the extrema. Again, piece of cake, right? And with the
coordinates of the extrema and the turning point, it is then also
easy to translate the yellow-green inequality into algebra. I leave that
as an exercise for you to complete in the comments. In the end the algebraic
counterpart to our yellow-green inequality pans out like this. Okay, in
exactly the same way the cubic having two solutions corresponds to equality
here and three solutions correspond to the reverse inequality. And so the sign
of the expression (q/2)^2+(p/3)^3 tells us how many
solutions our equation has. And this does not only work for negative p
roller-coaster cubics that we’ve been considering so far but for all cubics.
Right? For example, if p is positive, we have only one solution. But, also, with p
positive (q/2)^2+(p/3)^3 is positive. Works: one solution.
But there’s one tiny little annoying exception. Can you spot it? Well can you?
Well, when both p and q are zero, we still have
only one solution however in this case Q (q/2)^2+(p/3)^3 is
equal to 0 and so our table wrongly suggests there are two solutions. A minor glitch in the fabric of the
universe, which is easily fixed by talking for example about single and
repeated solutions. Anyway, the take away message from all this is that (q/2)^2+(p/3)^3 plays the same role for cubics as the
discriminant b^2-4ac plays for quadratics and just like b^2-4ac features prominently in the quadratic formula, will see that this
cubic discriminant takes center stage in the cubic formula. Remember when we wasted that day trying
to complete the cube? It turns out that time wasn’t
totally wasted. While playing with that perfect cube identity for hours we
noticed something very promising: the two middle terms have a common factor 3 u v.
If we pull it out, we get this. And now we can line up things like this. What
this shows us is that if we somehow find u and v so that all the lined up boxes
are equalities, then our cubic is effectively a perfect cube after all and
we will have solved our equation. What this amounts to is solving for u and v
in the green and yellow boxes. And then having done that we obtain our solution
x by just adding u and v. Pretty easy, actually. Let me just show you one way of
doing this. Well, first cube both sides of the green equation. Next, multiply the
yellow equation by v^3. Now we’ve got the same expression here and here and we
can sub like this. Shuffle everything to the left, as usual.
Looks bad at first glance, right? But it’s just a quadratic equation in
disguise. Think of the v^3 as the unknown.
There, blue squared plus something times blue plus something equals 0. Now
just use the quadratic formula to solve for the blue v^3 and cube rooting on
both sides gives this. Almost there. Remember that in our original equations u and v are indistinguishable and so if we solve for u, we get exactly the same
result. And now because of the plus/minus it would seem we can make two choices
each for both u and v, giving a total of three different ways to add them to get
solutions to our cubic equation: plus plus, minus minus, plus minus, and what
gives the same sum, minus plus. So here we have
three candidates for solutions AND cubic equations have up to three solutions.
That looks very promising, doesn’t it? Yes, it does. However, if we double-check
by substituting our candidates in the original equation, we find that only one,
the plus minus or minus plus gives us a genuine solution of the cubic. Why don’t
the other candidates for solutions work out? Can you figure out what the reason
might be? Let us know in the comments. Also, if all this only gives one of the
solutions, where are the other solutions hiding? Well, we’ll get to that. Why don’t they teach this? Well, there’s a
few related reasons that have to do with the way the cubic formula spits out
solutions. Remember that really nice example that I showed you at the
beginning. In that case, the solution given by the cubic formula easily
simplified down to a final answer of 6. Lucky with all those integer squares
and cubes materializing in just the right spots, wasn’t it? Of course it
wasn’t luck. One has to work damn hard to find an
example where the roots simplify that well. But things in general are even
worse than you might expect and some really weird stuff can happen. Let’s have
a look at another example. It’s pretty easy to guess a solution of this
equation. Can you see it. Ok, too late 🙂 Yep x=4 and just to check 4^3 that’s 64, 6 times 4 that’s 24, 64 minus 24 that’s 40, minus 40 is 0. Bingo!!
But what does our formula tell us? Plugging in -6 and -40 we get
this. So the cubic also has a monster rooty solution. But remember that 392
under the square root sign is our cubic discriminant and since 392 is positive
that tells us the cubic equation has only one solution. But that means that
the rooty monster in front of us must equal 4, the solution we guessed earlier.
Can you see at a glance that the two sides of this equation are really equal?
No, not obvious at all. It’s a challenge for you, can you do this from scratch? But
it can be even weirder. Have a look at this third example. Ok, plugging in -6 and -4 we get, well, under the square root sign our cubic discriminant
is 4 minus 8 that’s -4, which is negative. Hmmm, I hear you think. But let’s
first just focus on a negative discriminant which tells us that our
equation has three solutions. Let me just show you those solutions.
But now have a closer look at the expression spat out by our cubic formula.
Not only does this expression not look anything like those three solutions. In
addition, it contains square roots of negative numbers. This means that even
though our three solutions are real numbers, our formula expresses these
solutions in terms of complex numbers. How crazy weird is that. Not surprisingly
Cardano and his buddies were completely weirded out by this. They barely had an
understanding of negative numbers and complex numbers were way beyond their
imagination. And as it’s clear from the formula, the case of three roots always
leads to these weird complex number infested outputs. This horror case came
to be referred to as the casus irreduciblis. In fact, the struggle to
overcome the casus irreducibilis led to the discovery of complex numbers by
Rafael Bombelli not long after Cardano published the cubic formula.
I’ll make sense of the casus irreducibilis in the next chapter. First, let’s
just get all the cubic formula weirdness out in the open. How much weirder can it
get? Well, let’s see. When you’re only dealing with real
numbers, as we have until now, in the first instance it makes sense to speak
of THE cube root of this number, the one real cube root of this number.
However, when we’re dealing with non-real complex numbers,
there is no distinguished cube root anymore. Just as every nonzero complex
number has two square roots, every nonzero complex number has three cube
roots and none is distinguished in any way that makes it qualify for the job of
THE cube root. As we shall see this means that the expression up there does not
just stand for one of the real solutions down there, but for all three of them. One
final bit of weirdness. So Cardano’s formula outputs tame
integer, rational, etc. solutions in weird complex ways. That prompts the question
whether there is another totally different cubic formula that avoids this
weirdness. Right, there’s really no reason why there should not be several cubic
formulas. The answer to this question is both `no’ and `yes’. If by formula we mean a
combination of the coefficients using just basic arithmetic and square roots
and cube roots and the like, as an our cubic formula, the answer is `no’. Any such
formula cannot totally avoid the complex weirdness. This can be proved using a
heavy weapon called Galois theory. We skirted Galois theory in our previous
video on impossible constructions and it will also be the topic of a completely
insane, promise, completely insane Mathologer video in the near future. On the
other hand, if we allow ourselves to also use a bit of trigonometry, then we can
make up cubic formulas that always give real solutions in terms of real
expressions, avoiding the detour into the complex numbers. Anyway, to summarize,
except for the overcomplicated output of simple solutions, our cubic formula is
well-suited to dealing with cubic equations that have positive or zero
discriminant. That is, for equations that have exactly one or two solutions we
don’t need to enter the complex wilderness. Okay,
and that just leaves us with making sense of complex infested outputs in the case
of negative discriminates. Here’s one of the complex monsters
we created earlier. Let’s try to make sense of it. To begin we’ll just cross
our fingers and go for it without worrying too much about any worrying
details. Again, if you’ve never seen complex numbers, please just run with it
as best as you can. So we can always rewrite the square root in terms of the
square root of -1, like this. Now the square root of -1 is usually abbreviated
by the letter i. Marty behind the camera is shaping to throw something at
me. Okay, yes there’s a large nit you can pick here but please save your nitpicks
and projectiles for the comments in the end. It all works out ok. The two complex
numbers under the cube root only differ by the plus or minus sign in the middle.
That means they are complex conjugate numbers and so in the complex number
plane we picture them on opposite sides of the real axis. These two complex
numbers are also pinned down by this distance and this angle. That means we
can rewrite our complex numbers in terms of sine and cosine like this. There,
beautiful ! Ah, so nice. This way of writing complex
numbers is called the polar form. To find the roots of complex numbers written in
this form is very easy. First take the cube root of the distance like so.
Actually it simplifies quite nicely here. And then divide the angle by 3. Ok, there
roots coming up. Now, when we add up the two complex numbers up there, the
plus/minus imaginary parts cancel out, leaving us with a real number. That’s one
of our real solutions. But that’s not all. So far we’ve only used one cube root
each for the two terms in Cardano’s formula. There are two more each. All
these roots together from the corners of two equilateral triangles with centers
the origin. There’s one and there’s the second triangle. In total there are three
conjugate pairs among our six cube roots. Add up these conjugate pairs to get all
three solutions of our cubic equation. And just in case you’re wondering, and
this is a little trig challenge for you. Not `trick’, `trig’ 🙂 these numbers are
equal to these nice expressions. Great stuff, don’t you agree? I have to
admit that these videos are always a killer to put together but they are also a
blessing. While preparing them I always end up stumbling over lots of great
maths and amazing history of which before I only had the barest inkling,
always enough for another video – which I know I’ll never live long enough to
get to. So, in lieu of a second video here are just three fun facts
I stumbled across while working on this video, without explanation. Feel free to
give proving them a go in the comments. Fun fact number one. Here is a cubic with
three zeros. Draw in the tangents at those zeros. The tangents intersect the
graph in one more point each. Then those three points lie in a straight line.
Always! Nice, huh? Fun fact two. This is again about cubics with three zeros. Highlight
the inflection point. Draw an equilateral triangle whose centre is hovering
somewhere above the inflection point. Then it is always possible to rotate and
scale this triangle so that the three corners end up above the zeros. Pretty
neat. You can prove this easily based on what I said in the previous chapter. But
there’s more. Inscribe a circle into the triangle. Then the two extrema line up
with the left- and rightmost points of the circle like this. Final related fun
fact and this one is a real killer: Start again with a cubic polynomial but this
time the coefficients can be any complex numbers. So in general the three zeros of
such a polynomial will be three points in the complex plane, forming a triangle.
Highlight the midpoints of the edges of this triangle. It turns out there’s
exactly one ellipse that touches the edges in those midpoints. And here’s the
killer fun fact three: the two zeros of the derivative of our polynomial are the
focal points of this ellipse. This amazing result is called Marden’s theorem.
And one more thing is true. The zero of the second derivative, which is also the
z-coordinate of the “complex inflection point” is the center of the ellipse. And, phew, that’s just about it for today,
to finish I’ll just show you an animation of a method for solving quartic
equations. This solution is due to Ludovico Ferrari Cadano’s assistant. One of
the super surprising and famous results in algebra is that Ferrari’s method is as
far as we can go when we are asking for solutions in radicals.
There is no quintic formula of this type or a formula for anything beyond. As I
said proving this will be the mission of a future Mathologer video. For now enjoy
Ferrari and the animations and the music. Until next time 🙂

Reynold King

100 Replies to “500 years of NOT teaching THE CUBIC FORMULA. What is it they think you can’t handle?”

  1. Hi Mathologer, Can you please write a formula for the below Fibonacci expression.
    Basically the fixed number plus 10 percent of itself plus 10 percent of the number above it, equals the number below the number you started with, in a decimal form. Must be in a 5 column sequence. Being able to find a vertical answer, I would like to invert the formula to apply it and assume the same numbers be inverted before zero with a negative expression. Forgive the lack of knowledge, I'm no math expert of such. Not even amateur!

    https://www.youtube.com/watch?v=mRaAe6SswSE

  2. Actually, everything I've seen has m and n in the reduced cubic and under the radicals, with p and q equal to the cube roots.
    No big deal I suppose.

  3. Is there an algorithm or generalized formula (formulae) that yields any n-dimensional to the exponent n formula? Geometrically I can visualize that it should exist. However, you said that a quintic formula can be proved to not exist.

  4. I simply use sythetic division method to convert the equations into quadratic form and find roots 🙂 no need 2 memorise that formula anyway

  5. 31:30 You haven’t explained how these equilateral triangles arise. They come about from dividing the angle by 3: you can add or subtract any number of complete circles (2π radians) to/from the original angle and it is still the same, but when those complete circles are divided by 3, they offset the resulting angle by ±2π/3 or ±120°, giving you the 3 vertices of the equilateral triangle.

  6. Something occurs to me: your derivation of the cubic formula requires some knowledge of differential calculus. But this was only developed by Leibnitz and Newton in the 17th century; it was not known in the 16h century. So how did Tartaglia and Del Ferro figure it out without calculus?

  7. This YouTube video, https://www.youtube.com/watch?v=NMiO1iMiAUg, Cubics Equations Part 1 – solving Cubic Equations, presents this from a visual point of view… Do you think Cardano and/or Tartaglia likely visualized this problem visually, perhaps as suggested in this video, or do you think they stumbled on the solution through algebraic manipulation?

  8. So, I started a comment about why I find, not only the cubic formula to be redudant, but also about how I find the quadratic formula, mostly, redundant, but ended up going on a rant about everything I think is wrong with the education system so… well, im just not gonna bother with writing down the reasoning, ill just get tracked again.

  9. Huh, I learned the cubic equation in my Precalculus – Honors class. It still remains, together with Descartes' Rule of Signs, among the few elementary results I don't know how to derive. I recall that it wasn't tough to get rid of the squared term, and that the other two roots were obtained simply by synthetic division and application of the quadratic. It was a fantastic time waster. We had one case of doing the quartic formula (mostly consisting of our teacher doing it with us as peanut gallery) to say that we covered it.

  10. Even though it is very interesting. I can find many reasons of not teaching it.

    1) the formula is not easy to derive
    2) it is next to impossible for pre calculus students to do cube roots by hand
    3) understanding complex solutions, mean having to know complex numbers. Which itself requires trigonometry.

  11. Scenario: Summation of two integer's square is equal to a single integer's square:
    Observation: {2n(n-1)}^2 + (2n-1)^2 = {2n(n-1)+1}^2
    Where, n = any integer
    Now, I can derive from it the summation of 3, 4 or up to "n" numbers of integer's square is equals to a single integer's square.
    The question is, formulate the Summation of two integer's cube is equal to a single integers cube.
    Answer please.

  12. If you wanted to write an equation for the triangle above the solutions and/or the inscribed circle based on the original equation what would you do how would you do it. What if that equation gave you a way to more easy to solve the equation. At the very least it might be a complicated mathematical puzzle that’d be fun to watch

  13. Hi Mathologer, Can you please write a formula for the below Fibonacci expression.

    Basically the fixed number plus 10 percent of itself plus 10 percent of the number above it, equals the number below the number you started with, in a decimal form. Must be in a 5 column sequence. Being able to find a vertical answer, I would like to invert the formula to apply it and assume the same numbers be inverted before zero with a negative expression. Forgive the lack of knowledge, I'm no math expert of such. Not even amateur!

    https://www.youtube.com/watch?v=mRaAe6SswSE

  14. I am an undergraduate student from India. Please suggest a mathematics journal that deals with linear algebra/abstract algebra /real analysis… I can't understand the journals I saw online.

  15. Gottesdienst! Lovely music, and lovely site! Thank you so much. I pursue maths not too seriously in my old age and to help me kids, and this cubic (which I also never had, despite having a fair bit of math) I also never had. Veilen Danke!

  16. Good stuff. Good production values.

    Almost makes me want to teach high-school algebra. They've got a whole year. You could definitely give this stuff some time.

  17. We still use the radical sign to denote the principle cube root, and the convention is the root whose argument is 1/3 the argument of the radicand is the principle 3rd root.

  18. I have to take my hat off to mathematicians , I can do a few simple sums but these people are on a level that can only get glimpsed by standing on the shoulders of giants !😲

  19. BTW – the quintic equation is not unsolvable by any means…

    As I just solved it!

    Your videos are top-notch – they 're just the best there is!

    Thank you alot – it has been extremely helpful to me with ideas & true inspiration!

    I had a great time watching and rewatching this cubic video several times – you 're the best! 🙂

  20. mathologer, i'd write a comment in a 2 years old video by you…
    so i don't know if that will NOTICE YOU so ¿Can i put my comment back here?
    (Video of "Smale's paradox")
    —————————- copy starts here ——————————————-

    but… ¡it was easy than breath!
    Take a sphere
    Take a neddle (With hook the end needs to be like J)
    insert the neddle in one emisphere (For example… SOUTH, like the earth)
    CROSS ALL WAY TROUGH THE SPHERE
    STAB it again with the neddle
    now the hook does the rest
    PULL BACK all way through (Passing the north pole trough the HOLE thats makes the needle, in the south… the FIRST HOLE to understand us, so is BETTER IF THE HOOK IS TINY, almost invisible… but a hook)
    when it looks like a FOOTBALL/RUGBY "Ball"…
    QUIT THE NEDDLE
    and push (If not elastic) back to it's form

    ¡¡¡DONE!!!
    ¿Was that hard, really?

    ¡¡Chaoo!! 😉

    Edit: the "Crease" is moving, beacuse we DON'T move both emispheres at the same time… it means this "Turn out" is NOT SIMETRICAL, that all people are obsesed to do… not simetrical, you are just moving the SOUTH pole in this case, so, the crease, GOES from top to bottom UNTIL it Roll "Back" to the HOLE, and… turn out

    think in this move like a SOCKS, take the toes part "Pitching" it
    now PULL BACK
    ¿did see what happened to the "Open part"?
    yep, it turns inside out
    ¿But how?
    it ROLLS BACK INTO THE HOLE, like our sphere

    Easy

    ——————————- copy ends here ———————————

    and with no other things to say, i leave the comment section
    just a question ¿What do you think about my formula?

    ¡¡Chaoo!! 😉

  21. Here is an intriguing question for you: How can maths be used to determine what will happen beyond the so-called event horizon of the technological singularity more than I am able to determine without maths?

  22. 16:03 – The fact that the x-transpose shift completely eliminated the x^2 term just blew my mind!
    I had to rewind that quite a few times!

  23. We were taught the cubic formula and the entire story behind it, but on tests we just used our graphing calculators for anything higher than a quadratic.

  24. Luckily I had a hot girl as a babysitter and a couple hot girl Math teachers in high school and I was the best student in their classez cuz they both liked me and I felt way better after Jill influenced my feelings086542148908754211790009t4 oh I wrote1fue tfue3799531157900863126800954223780085311y7890097532358900643so that is tfue in number sequence but don't tell the government workers I will be sharing the codes for all the other words next oxycontin6279095 four863227909 girl86426909 desk85328005 lamp752 et79743 oh katry Perry et79743 kill8424807 twenty842178 mind7422690 brain96422 who852247 are54268085 you8732875 people853136 he974286 has964236 got72458 a 1 head8521478 wound853236 you8642 two864247 are75325790 officially 8763237 someone765333 else's3237 problem775433578 because864323789 it75423688 didn't87542246 no9753135 scientists853257888 it886433 can't874226800 be77532368 i9742235 like8875333 hot887532 girls876543 kids775433 these753276422 days7643225 it's864222467999 Wednesday764337895 my763236 dudes764234 girls9753223 like8632246 her874322 are76432 smoking7753222 according774222 to334678974 david775443333 I97421278gave8754225789 clay76532236 a65314700 couple985323578 swigs8753222 and664212780 his657743357899324790 tall86322 friend8776320005211 bodin98543hotel81827532357998532226632223579996332 hotel81827532357998532275322 hide086432236 am77532478 with9865433222 katrina764223 go76325790 try975322478 the652268996 nurses8853227 office8753224679 weebs98643224 streamers98three65432 gravity75211790 christians764222790 don't6421367900 have7632790 nuts9753225890 we98542147 are7742258953 exiles8753235girl799 in985322 lime8842257900532 programs97532268 holes98532278 dug98642227 in772236 the889063223 past88532 collapse6531357900 building64322688i I66322789 means77543357 less886433479 inmaginary8864433boy graph98544324 cops564246890 tied4422690 up77432279 emilia874222578 clarke875323799 is87431890a hot986532236 iceland87543278 girl8743368 and87423 I hit87433336 things876422 veryili75237906 easilyyyyyyyyyy875322467nd I could have but I didn't and then a few years later when I was out of high school my first hot math teacher and I we did and she is the best in the world!!!

  25. I've seen this general formula and the quartic immediately following in my father's college book, Lange's Handbook of Chemistry, Fouth Edition (September, 1941), appendix, Mathematical Tables and Formulas, page 7.

  26. Numerically, works well enough just to do all the algebra in the complex numbers, and pretend square roots of negative numbers are all fine. Then simply divide polynomial by the result, and do a lower order method on what's left. I just ran some tests with cubics and quadrics, and all the test cases were coming out to within the errors you normally expect. In addition, this is way more straight forward, compact, and debuggable than trying to copy over the general formula.

  27. That's absolutely amazing! I've always liked your videos, they are just amazing!
    I've always wanted to know how do you make your presentation, and what softwere do you use! I want to learn how to make the same presentations! Thank you very much for making those videos!

  28. Want to give props to my high school math teacher Gary Rubinstein for teaching the cubic formula, and so much more about math that I wouldn’t have learned with any other teacher.

  29. Great animations to help us SEE what is going on! Is there a systematic way of reducing the order of a high-order polynomial by dividing by a root that is easy to estimate? In grad school we used Numerical Recipes – it was a bit "cook book" once you'd figured out how to get "the answers". Many thanks!

  30. I don't properly know any calculus in a practical sense, but just knowing that "derivative" just means "rate of change of the function" and "second derivative" just means "rate of the function's rate of change changing" helps a lot watching this.

  31. Hi, I learned it in school in the GDR, in an extended math class in the begin of the 1970ths. It was rather interesting and fascinating.

  32. I am putting this on every video I watch and if you cack the code reply and say what it means 404551140092355961951865408669339474129449480207993863615775635180598144203723200395086916352672301398438057697530755218229287752919572007744552801129479163479060553498750275753411132869778819273052416402540354329817416696448090488803659567007163190457857699734130758643598274327335096931575895041675390942787873170661335722247298862045125791595426160773924676794122953143835662222031513452219751184412249138214449568348171899975774337102826858587881439491683894120751530203998099691128500509425834242534670920013404911360278528

  33. For 18:41 you could also multiply the y values of the stationary points to obtain a polarity that will tell you whether this cubic has 1,2,or 3 solutions 🙂

  34. I don't want to throw any projectile, but I would really like to know what happend to minus i at 29:50. Why is there no other solution if we insert minus i as square root of minus 1? This way, under both qubic roots could be 2 + 2i …

  35. Huh, we learned this up to Ferrari's equation for quartic / 4th order polynomials in highschool in Slovenia when I was going through school cca 20 years ago

  36. It's cool how the solutions for 3D differ by adding or subtracting i just like those for 2D differ by a either adding or subtracting a root

  37. La formule est encore plus facilement mémorisable si on l'apprend sous cette forme
    ∛ -Q ∓ √ Q²+P³
    si on écrit le polynome sous la forme: z³ + 3Pz + 2Q = 0

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