a) Let $0<x<1$ and $y=\left( {\rm sech}^{-1} x\right)^{3}$. Then the derivative

(Note: Look at the page for hyperbolic functions available in WeBWorK .)

$D_xy$ = .

b) Let $x>0$ and $z=\ln\left({\rm sech\,} x + \tanh x \right)$. Then the derivative

$D_{x}z$ = .