SOLUTION: the total area of a cylinder is 40pi. If h=8, find r.

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Question 1104057: the total area of a cylinder is 40pi. If h=8, find r.
Answer by greenestamps(13200) About Me  (Show Source):
You can put this solution on YOUR website!


Given that the height is 8, the total surface area is 2%28pi%29r%5E2%2B2%28pi%29rh+=+2%28pi%29r%5E2%2B16%28pi%29r

Given that the total surface area is 40pi,

2%28pi%29r%5E2%2B16%28pi%29r+=+40pi
2%28pi%29r%5E2%2B16%28pi%29r-40%28pi%29+=+0
r%5E2%2B8r-20+=+0 [dividing out the common factor 2pi]
%28r%2B10%29%28r-2%29+=+0

r = -10 or r=2; since this is the radius of the cylinder, we reject the negative solution.

The radius of the cylinder is 2.