If $f(x)=|\cos x|$, then $f'(3 \pi / 4)$ is equal to

$\frac{-1}{\sqrt{2}}$ $\frac{1}{\sqrt{2}}$ 1 none of these

$\frac{1}{\sqrt{2}}$

In the neighbourhood of $x=\frac{3 \pi}{4}$, we have $\cos x<0 \Rightarrow|\cos x|=-\cos x$ ∴   $f(x)=-\cos x $ $\Rightarrow f'(x)=\sin x$ $\Rightarrow f'\left(\frac{3 \pi}{4}\right)=\sin \frac{3 \pi}{4}=\frac{1}{\sqrt{2}}$