#### Answer

a) Need to calculate the value of inverse trigonometric function .
$cot^{-1}(-1)=\frac{3\pi}{4}$
b) Need to calculate the value of inverse trigonometric function.
$cot^{-1}({\sqrt 3})=(\frac{\pi}{6})$
c) Need to calculate the value of inverse trigonometric function.
$cot^{-1}(-\frac{1}{\sqrt 3})=\frac{2\pi}{3}$