# Homework Help: Finding volume in Polar Coordinates

1. Nov 2, 2013

### PsychonautQQ

1. The problem statement, all variables and given/known data
Find the volume of the wedge-shaped region contained in the cylinder x^2+y^2=9 bounded by the plane z=x and below by the xy plane

2. Relevant equations

3. The attempt at a solution
So it seems a common theme for me I have a hard time finding the limits of integration for the dθ term when I integrate in polar coordinates.

For this integral I am setting it up
triple integral rdzdrdθ where dr is between 0 and 3, dz is between 0 and rcosθ and dθ is between 0 and 2∏? is that correct?

2. Nov 2, 2013

### haruspex

You need to be careful with this one. There's no substitute (AFAIK) for visualising the region. Pay particular attention to the fact that it says below by the XY plane. That imposes a bound on z, and hence on theta.

3. Nov 2, 2013

### PsychonautQQ

does theta go from 0 to 45 since z=x it will create a 45 degree angle? dr between 0 and 3, and dz between 0 and rcos(theta)?

4. Nov 2, 2013

### LCKurtz

Show us a picture of what you have for the region.

5. Nov 2, 2013

### haruspex

Theta is an angle in the XY plane, so is not directly related to z = x.
If 0 <= z <= x and x = r cos θ and r > 0, what is the range of possible values for theta?

6. Nov 3, 2013

### vanhees71

Why should one use cylinder coordinates here to begin with?