Find the diameter of the base of a right cylinder if the surface area is 128(pi) and the height is 12cm.

1 answer:

8 0

$d=2r\\
A=128 \pi \\
A=2 \pi r^{2} +2 \pi rh \\\\
2 \pi r^{2} +2 \pi rh = 128 \pi \\
2 r^{2} +2rh = 128\\
2 r^{2}+24r-128=0\\
r^{2}+12r-64=0\\
delta=400\\
r_{1/2} = -16/4 \\
r=4cm\\
d=8cm\\
$

Radius / diameter as a length cannot be negative, that's why '-16' is wrong.

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