内圆柱的半径R,
(2-R):X=2:6,
R=2-X/3=(6-X)/3,
圆柱的侧面积S=2∏RX=2∏(6-X)X/3,
S=2∏(6-X)X/3
=(2∏/3)[9-(3-X)²]
=6∏-(3-X)²,
-(3-X)²