$$\iint_D \left( \frac{\partial P}{\partial x}+ \frac{\partial Q}{\partial y} \right) \, dx \, dy = \iint_D \operatorname{div} F \, dA$$
is the divergence theorem version of Green's Theorem I believe, but I was wondering how this is true? I feel like I need an intuitive idea of flux and divergence to figure out why this is true. I believe the unit normal of $∂D$ is $n = ( y'(t), −x'(t)) / \sqrt{[x'(t)]^2 + [ y'(t)]^2}$ also.