Question 443556
This problem is simply applying the area/circumference formulas for circles. The circumference C is 18pi, so


*[tex C = \LARGE 18\pi = 2\pi r]. This implies r = 9.


Given the area formula *[tex \LARGE A = \pi r^2], we replace r with 9 to obtain


*[tex \LARGE A = \pi (9^2) = 81\pi]


The circumference/area formulas are very useful to know. I usually advise knowing how to derive the formulas as well so that it does not become rote memorization. However you probably won't learn the proof of the area/circumference of a circle until you take a calculus course.