S = 2π(d/2)？ +(πd)*H = 4/d + πd？ /2

S ' = -4/d？ + πd = 0

d？ = 4/π

D = (4/ π) under the symbol of cube root is approximately equal to 1.084.

If the volume of a cylinder is 1, its surface area is the smallest when its diameter is 1.084.