ƒ' is decreasing in $\left( { - {\pi \over 2},0} \right)$ and increasing
in $\left( {0,{\pi \over 2}} \right)$
ƒ '(0) = ${ - {\pi \over 2}}$
ƒ is not differentiable at x = 0
ƒ' is increasing in $\left( { - {\pi \over 2},0} \right)$ and decreasing
in $\left( {0,{\pi \over 2}} \right)$