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## Multivariable calculus

### Course: Multivariable calculus > Unit 5

Lesson 9: Proof of Stokes' theorem# Stokes' theorem proof part 1

The beginning of a proof of Stokes' theorem for a special class of surfaces. Finding the curl of our vector field. Created by Sal Khan.

## Want to join the conversation?

- At around1:30, Sal mentions that z subscript xy = z subscript yx. Is this property called Clairaut's Theorem?(2 votes)
- That is Clairaut's Theorem. It states that if the partial second derivatives exist and are continuous, then the partial second derivatives are equal. This extends to higher order differentials as well.(3 votes)

- at2:40, where did the unit normal vector go? or doesn't it matter?(2 votes)
- Sal just forgot the vector sign over the dr. Because obiously it should be a scalar product and you need two vetors in order to compute a scalar product.(2 votes)

- are the surface area s and r same (excluding the height)(1 vote)
- prove that if the vector is the gradient of a scalar function then its line integral around a closed curve is zero.(1 vote)
- I need help studying for a test, it's on proofs can I have help?(0 votes)
- If you are talking about general induction proofs check out http://en.m.wikipedia.org/wiki/Mathematical_induction , otherwise look at geometric proofs athttp://www.sparknotes.com/math/geometry3/geometricproofs/ .

When you are talking about proofs you are probably talking about the latter.(1 vote)

## Video transcript

- [Instructor] In this video,
I will attempt to prove, or actually this and
the next several videos, attempt to prove a special
case version of Stokes' theorem or essentially Stokes'
theorem for a special case. And I'm doing this because the proof will be a little bit simpler, but at the same time
it's pretty convincing. And the special case we're
going to assume is that the surface we're dealing
with is a function of x and y. So if you give me any particular x and y, it only determines one
point on that surface. So a surface like this would be the case. So it's kind of a mapping of
this region in the xy-plane into three dimensions. So for any xy, we can
figure out the height. So essentially z is going
to be a function of x and y, and we can get a point on the surface. So this proof would not apply to a surface that's like a sphere
or something like that, where any point on the xy-plane would, could actually determine
two points on our surface, but this is a pretty good start. The other thing that
we're going to assume, we're going to assume that z, which is essentially
a function of x and y, that this function of x and y is, has continuous second-order derivatives, so continuous, continuous second, second derivatives. And the reason why I'm
gonna make that assumption is it's going to help us
in our proof later on. It's going to allow us to say that the partial of our surface or the partial of z with respect to x, and then taking the derivative
of that with respect to y, is going to be the same as the partial of z with respect to y and then taking the derivative
of that with respect to x. And in order to be able
to make this statement, we have to assume that z or our, the z right over here, z
is a function of x and y, has continuous second-order derivatives. And over here, we've just
written our vector field F that we're going to deal
with when we're trying to play with Stokes'
theorem, and we'll assume that it has continuous
first-order derivatives. Now with that out of the way, let's think about what
Stokes' theorem tells us. And then we'll think about,
for this particular case, how we can write it out, and
then hopefully we will see that the two things are equal. So let me write it out. So Stokes' theorem tells us that if, that F, F dot dr over some path, and the path that we care about is essentially this path right over here. I'll do it in blue. It's this path right over here. This is the boundary. This is the boundary of our surface. So this is c right over here. Stokes' theorem tells us that
this should be the same thing, this should be equivalent
to the surface integral over our surface, over our surface of curl of F, curl of F dot ds, dot, dotted with the surface itself. And so in this video, I wanna focus, or probably this and the next video, I wanna focus on the second half. I wanna focus this. I wanna see how we can express this given the assumptions we've made. And then after that, we're gonna see how we can express this
given the same assumptions. And then hopefully we'll
find that we get them to be equal to each other. So let's just start figuring out what the curl of F is equal to. So the curl of F, the curl of F is equal to, you could view it as the del operator crossed with our vector
field F, which is equal to, can write our components. So i, I'll do 'em in
different colors, i, j, and k components, i, j, and k components, and then I need to write my del operator or my partial operators I
guess I should call 'em. So the partial with respect to x, the partial with respect to y, partial with respect to z, and then I have to write
i, j, and k components of my vector field F. And I will do that in green,
well, I'll do it in blue. And so we have P, which
is a function of x, y, z, Q, which is a function of x, y, and z, and then R, which is a
function of x, y, and z. And so this is going to evaluate as, it's going to be i, i times, so blank out that column, that row. It's gonna be the partial
of R with respect to y, partial of R with respect to y minus the partial of Q with respect to z, minus the partial of Q with respect to z. And then checkerboard pattern minus j, gonna make that hat a little bit better, minus j and then times the
partial of R with respect to x, partial of R with respect to x minus the partial of P with respect to z, minus the partial of P with respect to z. And then finally, plus k, plus k, and that's going to be
times the partial of x with, or, sorry, the partial
of Q with respect to x, partial of Q with respect to x minus the partial of P with respect to y, minus the partial of P with respect to y. So we figured out the curl of
F, and I'll leave you there. I'm trying to make shorter videos now. And so in the next video, we have to figure out how to express ds, and then we'll just evaluate
this, this entire operation. We'll take the dot product.