Greens theorem on the circle $x^2 + y^2 = 16$. 
Use Greens theorem to calculate the area enclosed by the circle $x^2 + y^2 = 16$.

I'm confused on which part is $P$ and which part is $Q$ to use in the following equation
$$\iint\left(\frac{\partial Q}{\partial x}-\frac{\partial P}{\partial y}\right){\rm d}A$$

Hint: You want
$$\frac{\partial Q}{\partial x}-\frac{\partial P}{\partial y}=1$$
so the integral is
$$\iint_{x^{2}+y^{2}\leq 16}{\rm d}A$$
Can you find $P$ and $Q$ that satisfy this? Notice that there is more than one choice.
