1. The problem statement, all variables and given/known data How to integrate the double integral cos(x+y) dy dx from 0 to pi and from 0 to pi again. 2. Relevant equations The answer is -4. 3. The attempt at a solution Here's the work: u=x+y du=dy cos(u)du=sin(x+y) integral of [sin(x+y)] evaluate from 0 to pi dx from 0 to pi integral of (sin(x+pi)-sin(x))dx from 0 to pi And what's next?