Posted by Yuni on Friday, July 29, 2011 at 8:08pm.
It looks like you are being asked to use 22/7 for pi. In that case,
2(22/7)r^2 is the combined area of the two circular ends.
If the volume is V = pi*r^2*h, the curved cylindrical area is
2 pi*r*h = 2V/r.
That is where your 246000/r term comes from.
To minimize the surface area, set the derivative of A(r) equal to zero.
dA/dr = (88/7) r -246,000/r^2 = 0
r = (246,000*7/88)^1/3 = 26.9 cm
If you have not yet studied differential calculus, I suggest you graph A vs r and see where it is a mininum.
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