# Thread: Change of Variables in Multiple Integrals

1. ## Change of Variables in Multiple Integrals

Having a really tough time figuring out how to set this problem up:

Use the given transformation to evaluate the integral.

$\displaystyle \int \int_R x^2 ~dA$, where R is the region bounded by the ellipse $\displaystyle 9x^2+4y^2 = 36$;

$\displaystyle x=2u$

$\displaystyle y=3v$

2. Originally Posted by angel.white
Having a really tough time figuring out how to set this problem up:

Use the given transformation to evaluate the integral.

$\displaystyle \int \int_R x^2 ~dA$, where R is the region bounded by the ellipse $\displaystyle 9x^2+4y^2 = 36$;

$\displaystyle x=2u$

$\displaystyle y=3v$
$\displaystyle dA = dx \, dy \rightarrow J(u, v) \, du \, dv = [(2)(3) - (0)(0)] \, du \, dv = 6 \, du \, dv$.

$\displaystyle R_{xy}: ~ 9 x^2 + 4 y^2 \leq 36 ~ \rightarrow R_{uv}: ~ 36 u^2 + 36 v^2 \leq 36 \Rightarrow u^2 + v^2 \leq 1$.

So the double integral becomes $\displaystyle \int \int_{R_{uv}} (2u)^2 (6) \, du \, dv = 24 \int \int_{R_{uv}} u^2 \, du \, dv$.

To exploit the circular region in the uv-plane I'd suggest switching to polar coordinates now to seal the deal.