Which integral gives the area enclosed by the circle $x^{2} + y^{2} = a^{2}$?

Explanation

1. 01

   In the first quadrant the circle is $y = \sqrt{a^{2} - x^{2}}$, from $x = 0$ to $x = a$.

2. 02

   The four quadrants are equal, so the area is 4 times that integral, which is $\pi a^{2}$.