# Math Help - area; polar coordinates

1. ## area; polar coordinates

I have $\int\int_Rx^3+xy^2dxdy$,

$R$ = second quadrant of $1/4<=x^2+y^2<=1$

I changed $x^3+xy^2$ to polar coordinates:

$r^3cos^3(\theta)+r cos(\theta)r^2sin^2(\theta)=r^3cos(\theta)$

Do I evaluate the integral like this?

$\int_{\pi/2}^{\pi}\int_{1/2}^{1}r^3cos(\theta)drd\theta$

2. I think your boundaries are correct.

3. Don't forget about that extra r when using polar integration.

$x^{3}+xy^{2}$

$r^{3}cos^{3}{\theta}+rcos{\theta} \cdot r^{2}sin^{2}{\theta}=r^{3}cos{\theta}$

$r \cdot r^{3}cos{\theta}$

4. Should the answer be the same as $\frac{\pi-\pi \frac{1}{2}^2}{4}$ or am I missing the point here?

5. Or rather, that is R. Which is basically the upper left portion of a doughnut. But what does the area of integration look like?

6. Pardon me for being the village idiot, but could somebody please answer this one question for me:

When sketching the area of integration, do I just sketch $R$ , or do I have to consider $x^3+xy^2$ ?

7. Originally Posted by billym
Should the answer be the same as $\frac{\pi-\pi \frac{1}{2}^2}{4}$ or am I missing the point here?
The answer shouldn't have pi in it. $\int_{\frac{1}{2}}^1 \int_{\frac{\pi}{2}}^\pi r^4 \cos (\theta ) d\theta dr = \int_{\frac{1}{2}}^1 -r^4 dr$.

8. yeah I realized that last night. Anyway, I've got the right answer now. Thanks dudes.