Wikipedia definition of torsion

Question

I was looking at the Wikipedia definition (https://en.wikipedia.org/wiki/Torsion_of_a_curve) of the torsion of a curve where they claim that $\tau=\textbf{N}'\cdot \textbf{B}$. However, I don't see how they derived it from $\tau=-\textbf{N}\cdot \textbf{B}'$. Can anyone please explain how it is done? Thanks!

Answer 1

$N\cdot B=0$ identically since $B$ is a cross product where one of the factors is $N$. Now, differentiating and using the product rule gives $N’\cdot B+N\cdot B’=0$, so rearranging gives $N’\cdot B=-N\cdot B’$.