\[\left\|{\mathbf{T}_{u}\times\mathbf{T}_{v}}\right\|=\sqrt{\left(\frac{\partial%
(x,y)}{\partial(u,v)}\right)^{2}+\left(\frac{\partial(y,z)}{\partial(u,v)}%
\right)^{2}+\left(\frac{\partial(x,z)}{\partial(u,v)}\right)^{2}}.\]