# Calculating torsion of an asymptotic curve with nonzero curvature

I came across this problem when studying Gauss Map:

Show that $$\tau^2=-K$$ on an asymptotic curve. Here $$\tau$$ is torsion of the asymptotic curve $$\pmb r(u(s),v(s))$$(with curvature $$\kappa$$ nonzero) and $$K$$ is total curvature.

The only thing I can figure out is that since $$d\pmb{r}$$ is an asymptotic direction then principal curvature $$k_n=0$$. Also I saw another exercise which says in such situation we have: $$\tau=\frac{1}{\sqrt {EG-F^2}} \begin{vmatrix}(\dot v)^2 & -\dot u\dot v & (\dot u)^2 \\ E & F & G \\ L & M & N \\\end{vmatrix}$$ This one I could figure out by calculation. But I can't see any connection between these two expressions.

I know this question has been asked here. But I haven't studied the theorem mentioned in that answer yet. Anyone could give another solution? Thanks in advance!