
I'm slightly confused with this proof, from Stoke's Theorem we have:
$$\int_C \underline{F} \cdot \ d \underline{r} = \int \int_S (\nabla \times \underline{F}) \cdot \underline{n} \ dS$$ so going by all the using notation in the image we have:
$$\int_C \underline{F} \cdot \ d \underline{r} = \int \int_S \left (\dfrac{\partial F_2}{\partial x} + \dfrac{\partial F_1}{\partial y} \right ) \ dS$$ How did we get from $$\int \int_S\left (\dfrac{\partial F_2}{\partial x} + \dfrac{\partial F_1}{\partial y} \right ) \ dS$$ to $$\int \int_R\left (\dfrac{\partial F_2}{\partial x} + \dfrac{\partial F_1}{\partial y} \right ) \ dx dy$$