Find the dimensions of the rectangle of perimeter 36 cm which will sweep out a volume as large as possible when revolved about one of its sides.


The perimeter of the rectangle with length L and breadth b is 2(l + b)


Therefore,


2(L + b) = 36


L + b = 18


b = 18 - L


Let the rectangle be rotated about its breadth. Then the resulting cylinder formed will be of radius L and height b.


Volume of cylinder formed V = πL2b = π(18L2 - L3)


To find the dimensions that will result in the maximum volume:







L cannot be 0. L is taken as 12 cm.


Therefore b = 24.



At L = 12,



Therefore a maxima exists at L = 12, meaning the volume of the constructed cylinder will be maximum at L = 12 cm.


24