# Find Radius r of Cylinder?

A 39-ounce can of Hills Bros. coffee requires 188.5 square inches of aluminum. If its height is 7 inches, what is its radius?

[HINT: The surface area S of a right cylinder is S = 2(pi)r^2 + 2(pi)rh, where r is the radius and h is the height.]

### 1 Answer

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- 6 months agoFavorite Answer
188.5 = 2 * pi * r^2 + 2 * pi * r * 7

188.5 = 2 * pi * r^2 + 14 * pi * r

377 = 4 * pi * r^2 + 28 * pi * r

0 = 4pi * r^2 + 28pi * r - 377

r = (-28pi +/- sqrt(784 * pi^2 + 4 * 4 * 377 * pi)) / (8 * pi)

r = (-28pi +/- sqrt(16 * 49 * pi^2 + 16 * 377 * pi)) / (8 * pi)

r = (-28 * pi +/- 4 * sqrt(49 * pi^2 + 377 * pi)) / (8 * pi)

r = (-7 * pi +/- sqrt(49 * pi^2 + 377 * pi)) / (2 * pi)

r > 0

r = (-7pi + sqrt(49pi^2 + 377 * pi)) / (2pi)

r = 3.0000543669128084295473237405432

r = 3 inches

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