# The total area of a cylinder is 40. If height = 8, find the radius. (The formula for total area used in class is: Total area = 2πrh + 2πr²)

Friday, July 23rd, 2010

### Solution:

The total area is 40π=2πrh+2πr^2

=2πr*8 + 2πr²

=16πr + 2πr²

=16r + 2r²

If we rearrange the terms and divide the terms by 2 (in order to simplify the equation), we get:

r^{2} + 8r – 20 = 0

(r+10) (r-2) = 0

r = 2 or -10

Therefore, **r = 2** (assuming that -10 is not an option!).

Tags: 9th Grade, Geometry, Homework Answers, Math

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