# Homework Help: Finding area of integration in polar?

1. Apr 22, 2012

### um0123

1. The problem statement, all variables and given/known data

Im having trouble find the area of integration for this integral which i have to convert to polar:

$\int_0^2 \int_0^\sqrt{1-(x-1)^2} \frac{x+y}{x^2 + y^2}$ dydx
2. Relevant equations

x = rcosθ
y = rsinθ
r = x^2 + y^2
3. The attempt at a solution

i know exactly what to do to the integrand, i just dont understand how to turn the upper limit sqrt(1-(x-1)^2) into polar, i can't visualize it at all!!!

i know it eventually turns into 2cosθ but i don't understand how to get there.

2. Apr 22, 2012

### sharks

What is the whole question? Have you been given that double integral only? If you deduced those limits on your own, maybe you have them wrong.

3. Apr 22, 2012

### LCKurtz

$y=\sqrt{(1-(x-1)^2}$ is the top half of the circle $(x-1)^2+y^2=1$. Just convert that to polar coordinates and simplify it and be surprised.

4. Apr 22, 2012

### sharks

$y=\sqrt{(1-(x-1)^2}$ turns into $2\cos \theta$.

To help you to visualize the problem, convert the Cartesian coordinate integral into its polar form (i haven't included the integrand for simplicity, but you will obviously have to convert the latter as well):
$$\int^{\theta=\frac{\pi}{2}}_{\theta=0} \int^{r=2\cos \theta}_{r=0} rdrd\theta$$
Hence, you will realize that you need to express r in terms of $\theta$, which should simplify the polar conversion process.

Last edited: Apr 22, 2012
5. Apr 22, 2012

### LCKurtz

No it doesn't. Check your arithmetic.

6. Apr 22, 2012

### sharks

Yes, you are correct. I had substituted r=1 somewhere in the conversion.
I've edited my previous post as well.

7. Apr 22, 2012

### LCKurtz

Just a reminder from the rules:

NOTE: ... Once your question or problem has been responded to, do not go back and delete (or edit) your original post.