SOLUTION : $\because \cot \left(\cot ^{-1} x\right)=x, \forall x \in R \\$
$\therefore \quad \cot \left(\cot ^{-1} \frac{3 \pi}{4}\right)=\frac{3 \pi}{4} \\$
$ \operatorname{cosec}\left\{\cot \left(\cot ^{-1} \frac{3 \pi}{4}\right)\right\}=\operatorname{cosec}\left(\frac{3 \pi}{4}\right)=\sqrt{2} $