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<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)`