a) Let $f(x)=\frac {\displaystyle 1}{\displaystyle 3} \tan^{-1}\left(\frac {\displaystyle x}{\displaystyle 3}\right)$.

Find $f'(x)$ = .
b) Let $g(x)=\frac {\displaystyle x}{\displaystyle 2}\sqrt{9-x^2}+ 4.5\sin^{-1}\left(\frac {\displaystyle x}{\displaystyle 3}\right)$.

Find $g'(x)$ = .