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jmc

prealgebra intermediate

Problem

A circle with center and radius three inches is tangent at to a circle with center , as shown. If point is on the small circle, what is the area of the shaded region? Express your answer in terms of .

problem
Solution
Since the small circle is tangent to the large circle at and point lies on the smaller circle and is the center of the larger circle, we know the radius of the bigger circle is twice the radius of the smaller circle, or six inches.

To find the shaded area, subtract the area of the smaller circle from the area of the larger circle. . Consider the tangent line to circle at , say line . Then . But since circle is tangent to circle at , we also have that . Hence is on segment , and is a diameter of circle . Thus by homothety circle covers the area of circle . The shaded region is thus of the area of circle , and hence is 3 times the area of circle , or simply .
Final answer
27\pi