Step by step
The question
Question: use Green's theorem to find ∮ (x − y) dx + (x + y) dy around the rectangle 0 ≤ x ≤ 3, 0 ≤ y ≤ 2.
1 Find P and Q
The path goes once around the rectangle, counterclockwise.
The part before dx is P = x − y.
The one before dy is Q = x + y.
2 Find the inside
Green's theorem: the line integral around equals the double integral inside of ∂Q/∂x minus ∂P/∂y.
Term by term.
x gives 1.
y has no x: it is a fixed number, so it gives 0.
Drop the zeros: ∂Q/∂x = 1.
Term by term.
x has no y: it is a fixed number, so it gives 0.
y gives 1.
Drop the zeros: ∂P/∂y = −1.
Take away: 1 minus (−1).
So the inside is 2.
Careful!
The order matters.
Q by x, minus P by y.
The other way round changes the sign.
3 Find the limits
Over the rectangle: y from 0 to 2 inside, x from 0 to 3 outside.
4 The inner integral
Integrate each term: the power goes up by 1, and we divide by the new power.
Put in the top y = 2, then take away the bottom y = 0.
The bottom part is 0.
So it is 4.
5 The outer integral
Integrate each term: the power goes up by 1, and we divide by the new power.
Put in the top x = 3, then take away the bottom x = 0.
The bottom part is 0.
So it is 12.
6 Check
Let's check by walking the four sides, one at a time.
Bottom 9/2, right 8, top 3/2, left −2.
On each side, only one of x and y moves.
Added up: 12.
The same as the double integral.
Right!
Let's look back.
First: find P and Q.
Then: find the inside.
Then: find the limits.
Then: the inner integral.
Then: the outer integral.
Last: check.