Question

If f(x) = |cos x|, find f'`((3pi)/4)`

Answer 1

When `pi/2` < x , pi, cos x < 0` 

So that |cos x| = `- cos x`

i.e., f(x) = `- cos x`

⇒ f'(x) = sin x.

Hence, `"f'"((3pi)/4) = sin ((3pi)/4) = 1/sqrt(2)`