Frenet-Serret formula proof Prove that $$\textbf{r}''' = [s'''-\kappa^2(s')^3]\textbf{ T } + [3\kappa s's''+\kappa'(s')^2]\textbf{ N }+\kappa \hspace{1mm}\tau (s')^3\textbf{B}.$$
What is $\tau$, I can't figure that part out.
All ideas are welcome.

Actually its easy. You just need to start with an equation r'= s' t and take its two further derivaties w.r.t s, then you just need to use all the three Ferent-Serret equations in the whole process, wherever required.
Note: there will be a use of chain rule in the initial process.
