Question

I don't get this problem hell!

Answer 1

Take two concentric circle, one with radius $r$ and the other with radius $R$, and suppose $r$. (Picture a dartboard with two circular sections.)

The area of the smaller circle is $\pi r^2$. The area of the larger one is $\pi R^2$. So the area between the larger circle and smaller circle is $\pi R^2-\pi r^2=\pi(R^2-r^2)$