## Precalculus: Concepts Through Functions, A Unit Circle Approach to Trigonometry (3rd Edition)

$\dfrac{\pi}{2}$
We need to find: $\cos^{-1}(0)$ The inverse cosine function has the domain $0\le \theta \le \pi$. We are looking for the angle whole cosine value equals zero. We know from the unit circle that $\cos (\dfrac{\pi}{2})=0$ Since $\frac{\pi}{2}$ lines in the domain $0\le \theta \le \pi$, then we have: $\cos^{-1}(0)=\dfrac{\pi}{2}$