# Homework Help: Convert the double integral to polar coordinates

1. Dec 9, 2011

### tag16

1. The problem statement, all variables and given/known data
Evaluate the double integral by converting to polar coordinates.
∫∫ arctan y/x dA; R is the sector in the first quadrant between the circles 1/4= x^2+y^2 and x^2+y^2=1 and the lines y=x/√3 and y=x.

2. Relevant equations

arctan y/x= θ

3. The attempt at a solution

D={(r,θ) l 1/4≤r≤1, ∏/6≤∏/4}

∫∫ θr drdθ

Which gives me 25∏^2/3072 which is incorrect according to my book. The correct answer is 15∏^2/2304.

2. Dec 9, 2011

### Dick

3. Dec 10, 2011

### tag16

I'm not sure, I thought maybe 1/8 but that's not right.

4. Dec 10, 2011

### Dick

A circle around the origin has the equation x^2+y^2=r^2. If r^2=1/4, what's r?

5. Dec 10, 2011

### tag16

I tried 1/2 when I was originally having difficulty with the problem and still got the wrong answer. I guess I must be missing something else.

6. Dec 10, 2011

### Dick

Their given answer isn't in lowest terms for some reason. 15*pi/2304=5*pi/768. Is that part of the trouble?

7. Dec 10, 2011

### SammyS

Staff Emeritus
Try again, This gives the right answer using the rest of what you have.