A circle has its center at the origin and passes through (7, 2). What is its area?
Say point A = (7, 2). Draw radius OA, and construct the right triangle:

In triangle OAB, use the Pythagorean Theorem to find r2.
r2 = 72 + 22 = 49 + 4 = 53
Notice, we don't need to solve for r, because r2 appears in the formula:
A =πr2=53π