prove that if V $\subset \mathbb{R^3}$ and $C^{1}$. u,v :$\mathbb{R^3} \to \mathbb{R}$ are $C^{2}$, so, $$\iiint_V (u\cdot \Delta v+ \bigtriangledown u\cdot \bigtriangledown v)dxdydz=\iint_{\partial V}u\cdot \frac{\partial v}{\partial n}ds$$
when $\frac{\partial v}{\partial n}$ directional derivative of v direction of a normal unit to $\partial v$ turn out.
$\Delta v = v_{xx}+v_{yy}+v_{zz} , \bigtriangledown u=(u_{x},u_{y},u_{z})$
I tried to use Divergence theorem but I didn't know from what to start.
thank you,