# How do you evaluate  tan ^ -1 ( -1)?

${\tan}^{- 1} = - \frac{\pi}{4}$
As $\tan \left(\frac{\pi}{4}\right) = 1$ and $\tan \left(- \theta\right) = - \tan \theta$
$\tan \left(- \frac{\pi}{4}\right) = - 1$
and ${\tan}^{- 1} = - \frac{\pi}{4}$