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)
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:
r2 + 8r – 20 = 0
(r+10) (r-2) = 0
r = 2 or -10
Therefore, r = 2 (assuming that -10 is not an option!).
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