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3. At a zoo, the lion pen is surrounded by a ring-shaped sidewalk. The outer edge of the sidewalk is a circle with a radius of 11 m. The inner edge of the sidewalk is a circle with a radius of 9 m.

(a) Write and simplify an expression for the exact area of the sidewalk.
(b) Find the approximate area of the sidewalk. Use 3.14 to approximate .