Solution:
$\sec ^{-1}\left(\frac{-2}{\sqrt{3}}\right)=\pi-\sec ^{-1}\left(\frac{2}{\sqrt{3}}\right)\left[\right.$ Formula: $\left.\sec ^{-1}(-x)=\pi-\sec ^{-1}(x)\right]$
$\begin{array}{l}
=\pi-\frac{\pi}{6} \\
=\frac{5 \pi}{6}
\end{array}$
Find the principal value of each of the following : $\sec ^{-1}\left(\frac{-2}{\sqrt{3}}\right)$
Find the principal value of each of the following : $\sec ^{-1}\left(\frac{-2}{\sqrt{3}}\right)$