i need a proof for the derivate of the normal vecture of curves.
$n'(s)= -kt - \tau b $ ,
where k is the curvature, t is the speed vektor and $\tau$ is the torsion, and $n'(s)$ is the derivate of the normal vecture, and b the binormal vector.
i saw proofs yet, but they arent such detailed as i could understand. could please someone give me a detailed proof for that formula?