Answer
Zero.
Work Step by Step
The function $cosx$ on the domain interval $x:[0,\pi]$ has two separate sections where it changes its position in respect to the x-axis. The function on $[0,\frac{\pi}{2}]$ is positive and the function on $[\frac{\pi}{2},\pi]$ is negative. Therefore, the positive section counts as positive area and the negative section is negative area. The two sections are discovered to have the same area, so they cancel out and the integral is 0.
