Answer
sample:
$a=\displaystyle \frac{\pi}{2},\quad b=\frac{5\pi}{2}$
Work Step by Step
Graphing $f(x)=\cos x$,
we want an interval where half the graph is above the x-axis, and the other half is below...
For example: $[\displaystyle \frac{\pi}{2}, \frac{5\pi}{2}]$
