 Solution

The vectors $T={\partial }_{\tau }x$ and $S={\partial }_{\sigma }x$ are coordinate vector fields on the parameter surface, so their ordinary commutator vanishes. Torsion-freeness then givesSince each $\tau$-curve is geodesic, ${\nabla }_{T}T=0$. Apply the Ricci identity to $V=T$ and use ${\nabla }_{S}T={\nabla }_{T}S$:Thus the desired form holds with