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## Homework Statement

Hi there! So I'm working on an old homework problem for review so that I actually have the solution, but I'm not sure how to compute a certain part. Here it is:

$$\int \int rcos^2(\theta)dr - r^2cos(\theta)\sin(\theta)d\theta$$

## Homework Equations

The solution involves (what I think is) a rewrite of $$dr$$ in terms of $$d\theta$$, but I don't follow the substitutions the author makes.

## The Attempt at a Solution

Any thoughts on how to approach a problem like this? Thanks!