Find the principal value of each of the following :

$\tan (-1)=-\tan (1)$ [ Formula: $\tan ^{-1}(-x)=-\tan ^{-1}(x)$ ]

[We know that $\tan \frac{\pi}{4}=1$, thus $\tan ^{-1} \frac{\pi}{4}=1$ ]

$=-\frac{\pi}{4}$