a) Let $\displaystyle | x | < \frac{\pi}{2}$ and $y=\left(\cos^{-1}x\right)^{3}$. Then the derivative

$D_{x}y$ = .

b) Let $\displaystyle |x|<\frac{\pi}{2}$ and $z=\ln\left(\sec x + \tan x \right)$. Then the derivative

$D_{x}z$ = .