How much greater, in square inches, is the area of a circle of radius 20 inches than a circle of diameter 20 inches? Express your answer in terms of $\pi$.
First circle: pi(20)^2=400pi inches squared:
Second cirle"pi(20/2)^2=100 pi inches squared.
Thus, 400pi-100pi=300 pi inches squared(square inches).
+-291 
