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February 12, 2016
Posted by **sonia** on Sunday, April 28, 2013 at 3:47am.

- math -
**Reiny**, Sunday, April 28, 2013 at 12:08pmradius -- r

height --- h

given: πr^2 h = 21

h = 21/(πr^2)

Surface area (SA) = 2πr^2 + 2πrh

= 2πr^2 + 2πr(21/(πr^2)

= 2πr^2 + 42/r

d(SA)/dr = 4πr - 42/r^2

= 0 for a max/min of SA

4πr = 42/r^2

r^3 = 42/(4π)

r = (42/(4π)^(1/3) = 1.495163.. or appr 1.5

then h = 21/(πr^2) = 2.99027 or appr 3

notice the h = 2r