# If $f(x)=\mid \cos x\mid$,then $f'(\frac{\pi}{4})$=____________.

Toolbox:
• The derivative of a function $f(x)$ is $f'(x)$.
• $f(x)=\cos x\Rightarrow f'(x)=-\sin x$
• $f(x)=\sin x\Rightarrow f'(x)=\cos x$
• $\sin\big(\large\frac{\pi}{4}\big)=\large\frac{1}{\sqrt 2}$
$f(x)=\mid \cos x\mid$
$f'(x)=\mid -\sin x\mid$
$\sin\big(\large\frac{\pi}{4}\big)=\large\frac{1}{\sqrt 2}$
$f'\big(\large\frac{\pi}{4}\big)=-\large\frac{1}{\sqrt 2}$