If $\sin^{-1} x = \frac{\pi}{5}$ for some $x$ in $[-1, 1]$, what is $\cos^{-1} x$?

Explanation

1. 01

   $\sin^{-1} x + \cos^{-1} x = \frac{\pi}{2}$ for every $x$ in $[-1, 1]$.

2. 02

   $\cos^{-1} x = \frac{\pi}{2} - \frac{\pi}{5} = \frac{3\pi}{10}$.