And what is the radius of the coin? Well, the radius of the coin is this height, is this height right over here. And what is that height? Well, it is one minus f of x. So that is equal to the radius. So the area, the area of kind of the face of this coin is going to be pi, the area of the face of this coin is going to be pi times the radius squared, which is equal to pi times one minus f of x, one minus f of x squared. That's this blue area right over here.