I need to find a derivative of follow equation $$ \left(r_{0} + \sum [a_{i}\cos(i\phi) + b_{i}\sin(i\phi)] \right)({\sin\phi-k\cos\phi}) - b = 0 $$
I know the derivative of $\left(r_{0} + \sum [a_{i}\cos(i\phi) + b_{i}\sin(i\phi)] \right)$. Let this part is $X$ then I need to solve follow equation:
$$ (X({\sin\phi-k\cos\phi}) - b)' = 0 $$ $$ X' · ({\sin\phi-k\cos\phi}) + X · ({\cos\phi+k\sin\phi}) = 0 $$
Is that correct?