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.