Question

Write the value of $\cos ^{-1}\left(\cos \frac{7 \pi}{6}\right)$.

Answer 1

We have $\cos ^{-1}\left(\cos \frac{7 \pi}{6}\right)=\cos ^{-1}\left[\cos \left(2 \pi-\frac{5 \pi}{6}\right)\right]=\cos ^{-1} \cdot \cos \frac{5 \pi}{6} \quad=\frac{5 \pi}{6}$