Question

find the derivative

Answer 1

i know i should use product rule

Answer 2

still then i dont the answer

Answer 3

\[f(x)=3sec^2(\frac{x}{3})=\frac{3}{cos^2(\frac{x}{3})}=3cos^{-2}(\frac{x}{3})\]
You can use chain rule now. cos^2(x)=(1+cos(2x))/2