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

In the neighbourhood of $x=\frac{3 \pi}{4}$, we have

$\cos x<0 \Rightarrow|\cos x|=-\cos x$

∴   $f(x)=-\cos x $

$\Rightarrow f'(x)=\sin x$

$\Rightarrow f'\left(\frac{3 \pi}{4}\right)=\sin \frac{3 \pi}{4}=\frac{1}{\sqrt{2}}$