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# 𝘶-substitution intro

AP.CALC:
FUN‑6 (EU)
,
FUN‑6.D (LO)
,
FUN‑6.D.1 (EK)
Using u-substitution to find the anti-derivative of a function. Seeing that u-substitution is the inverse of the chain rule. Created by Sal Khan.

## Want to join the conversation?

• In these series of videos (U-substitution) you introduce the treatment of the derivative operators (dx, du, etc) as fractions. You specify that they really are not, but treat them like that anyway. Have you made a video where you prove, or at least explain why they can be treated like that? Are there also other nice ways you can treat the operators? Maybe there is a video that introduces all the notations in differential form?
• the dxs and dus can actually be thought of as an infinitesimal quantity (like Leibniz did) as well as a marker for the variable you are dealing with. This is because the d ultimately derives from the little delta meaning difference (in differential calculus) that eventually becomes negligibly small. This d is actually subject to multiple interchangeable definitions that can be very useful like a very small area of value one.
• I am still confused about what happens to the du. I understand the dx portion but, how exactly is the du taken out of the equation.
• Imagine they are fractions. dy/du has du on the numerator and du/dx has du on the numerator. Multiplying them together, you get (du/du)*(dy/dx), and since du/du is 1, you are just left with dy/dx.
• Sal said that you could treat du/dx as a fraction and in the later videos, he does that. But, isn't du/dx not really a fraction? Is there a proof for what Sal does in the video (du/dx=3x^2+2x --> du=3x^2+2x dx)?
• Actually, du/dx is a fraction! It's a bit different, though.

Derivatives come from the formula of slope:
(change in y)/(change in x)
When we take the derivative of y, we get dy/dx, or:
(infinitely small change in y)/(infinitely small change in x)
If we use technical terms, this is:
(differential of y)/(differential of x)

NOTE: dy/dx can change from x to x with the same function because while y changes its speed, x doesn't because it's the independent variable, meaning that dy changes but dx doesn't, changing dy/dx.

Back on topic, though, d(variable) simply means a differential of that variable. It is a quantity, but a very small one (an infinitesimal one).

Since differentials are technically quantities, so then dy/dx is just a fraction of quantities! Just like any fraction, you can multiply it by its denominator to leave just the numerator!

I hope this helps!
• At Sal says that he will go more in depth in an other video. Does anybody know where this video is?
• why do we don't consider dx or du in the final antiderivitive answer? if they are concerned in definite integral, then they should also be considered in antiderivites also, but it's not like that,,,,Why??
• When we integrate with respect to a variable such as x or u, the corresponding differential, dx or du, seems to disappear. But it doesn't truly disappear: it gets integrated into the final answer.

This is easiest to see with the definite integral. When you go back to the way Riemann sums work, you see that dx, an increment in the x direction, gets multiplied by a quantity in the y dimension to produce the result. Note that dx does not appear in the result, which is simply a constant, but dx was used in producing that result.

The fundamental theorem of calculus tells us that we can produce the same result that we get from taking the limit of Riemann sums much more easily by finding the antiderivative of a function. The dx has been incorporated into the result, so we don't show it separately.
• If you were subbing for the most imbedded part of the function but what you subbed also existed somewhere else in the function, sometimes it doesn't work to substitute for both. Does it break algebraic rules to selectively substitute for one and leave the other as is?
• Sorry, that is ordinarily not a valid means of substitution. A valid substitution, generally speaking, requires that ALL references to the original variable be replaced ESPECIALLY including its dx (or whatever the variable is). You cannot have ANY stray bits leftover.

The purpose of u substitution is to wind up with ∫ f(u) du
Where f(u) du is something you know how to integrate. And remember du is the derivative of whatever you called u, it is NOT just some notation.

So, the answer is, no, you cannot do u-substitution that way. With integration, being close to a standard form is not good enough: you must have an exact match. For example,
∫ (x)∙cos(x²) dx is very easy to integrate but the very similar looking
∫ cos (x²) dx is nightmarishly difficult (getting into something called Fresnel integrals).

It isn't really an exception, but you can sort of have leftover bits when you do integration by parts. In that case, you do two u-substitutions (but you call them by different variables). However, even with integration by parts taken together the two substitutions must completely replace the original variable, with no leftovers allowed.
• ʃ f(x).g(x) dx we can`t say that this expression is
ʃ f(x)dx . g(x) so how we could say that
ʃ (3x3 + 2x) . e(x3 + x2 ) dx is equal to the expression
ʃ (3x3 + 2x) dx . e(x3 + x2 )
• You're right, ( ʃ f(x)*g(x)*dx ) is different from ( ʃ f(x)*dx )*g(x), but what Sal meant was ( ʃ f(x)*dx*g(x) ). It's a product, so you can change the order of the terms multiplied together.
• Why do we use u? Is it just tradition?

Also, if you see something like ∫(dx)∙e(x), why don't you just cancel out ∫(dx)? The sum of the little parts of x (Calculus Made Easy by Martin Gardner and Sylvanus B. Thompson say that's how you can think of it) must equal x, after all...
• This is the u-substitution introduction:

"U-substitution is a must-have tool for any integrating arsenal (tools aren't normally put in arsenals, but that sounds better than toolkit). It is essentially the reverise chain rule. U-substitution is very useful for any integral where an expression is of the form g(f(x))f'(x)(and a few other cases). Over time, you'll be able to do these in your head without necessarily even explicitly substituting. Why the letter "u"? Well, it could have been anything, but this is the convention. I guess why not the letter "u" :)"

Do you agree that this is what an indefinite integral does?
int(y'dx)=y
If so, do you agree that y' equals dy/dx?
int((dy/dx)dx)=y
If so, do you agree that the (dx)s cancel out it the integral?
int(dy)=y
The integral sums up all the differentials of a variable and makes it that variable. I think that's Martin Gardner and Sylvanus B. Thompson were talking about. You can't just get rid of the dx because it needs to cancel out the the dx in the derivative of the function.

If you were referring to something different than what I just showed, please tell me by commenting on this answer.
• I have a question. How do you know on what does the integral symbol apply? I always assumed that if I saw an integral sign I had to integrate the part that was between the integral symbol and the dX. But in this example at the e^(bla) part also has to be integrated, while it's behind the dx.

For example in this video; https://www.khanacademy.org/math/calculus/integral-calculus/indefinite_integrals/v/antiderivative-of-hairier-expression everything in between the integral sign and the dx has to be integrated. But if dx is a factor of the expression behind the integral sign then this doesn't apply.

Can somebody clear up for me, how I know what is part of an integrand and what not?

Many thanks.
• While it is customary to put the dx at the end, it is not required.
So, you integrate everything after the ∫ symbol. If there is anything that you need to treat separately you would typically have the problem written with some clear distinction indicated such as setting the integrand off with { }. But, usually if there is something not to be integrated, you would put that before the ∫ symbol.
• I'm kinda loss on this integral thing, I understand that the integration by parts is for multiplying functions and understand completely the antiderivative process , however on this u substitution I don¡t quite get the intuition behind this. When should I use u substitution ? Also, on the following exercises they use something called reverse chain rule, and on the hints they provide the formula f'(g(x))*g'(t)dx = f(g'(t)) + C , and yet I can't figure out where are they getting that from, we were never given an specific formula for u-substitution... Thanks to all willing to help ! (:
• Anti-derivatives are harder than derivatives. While derivatives tend to be evaluated exactly, using a far more limited set of rules. Anti-derivatives require many more rules and methods to evaluate if it can be evaluated in terms of an exact function. There are a bunch of different methods and approaches to solve integrals, sometimes multiple methods can be used, sometimes only one can solve it exactly, and sometimes exact equations don't exist leading to numerical approximations. u substitution is another method of evaluating an integral in an attempt to transform an integral that doesn't match a known integral rule into one that does. I'll give an alternative formulation of the rule.

u = g(x), if the range is some interval and f(x) is continuous over that interval then,
integral(f(g(x))g'(x)dx) = integral(f(u) du), say you have some function x * sin(x^2+5) that you want to evaluate the integral of using this method.

Re-arrange that function to sin(x^2+5) * x, now the derivative of the function in the sine function looks remarkably like x, constant values are just fine in the formulation so.
x dx = 1/2 * du giving a straightforward way to define this, u is x^2+5, since there isn't a 2x value it is known there will be a 1/2 factor. the substituted integral would be 1/2 * integral(sin(u)*du) = - 1/2 cos(u) + C = -1/2 cos(x^2+5) + C.

The basic idea is try to try to evaluate a complicated integral using a more simple one, determining u depends a lot on intuition, experience, and trial and error, than an exact equation.