英语翻译
A potter has a fixed volume of clay in the form of a cylinder laying on its side.As he rolls the clay,the length of the cylinder,L,increases while the radius,r,decreases.If the length of the cylinder is increasing at a constant rate of 0.2 cm per second,find the rate at which the radius is changing when the radius is 1.5 cm and the length is 4 cm.