differentiation

A = x^2/16 + (10 - x)^2 / 4π

dA/dx = x/8 - (10 - x)/2π

d^2A/dx^2 = 1/8 + 1/2π

Set dA/dx = 0

x/8 - (10 - x)/2π = 0

πx - 4(10 - x) = 0

x(π + 4) = 40

x = 40/(π + 4)

For x = 40/(π + 4), d^2A/dx^2 = 1/8 + 1/2π > 0

So, minimum value of A is attained when x = 40/(π + 4)

The min value of A

= [40/(π + 4)]^2/16 + [10 - (40/(π + 4))]^2/4π

= 3.5 (2 d.p.)