The proportional coefficient of the construction cost per square meter to the square of the number of buildings is k.

k*5^2* 1000*5+5000000= 1200* 1000*5

K=8

s=(5000000+8*x^2* 1000*x)/( 1000*x)

=5000/X+8X^2

F(X) = 5000/x+8x 2, f (x) first ↓ then =

Because X is an integer, when X = 7, S is the smallest, and S = 1 106.28 yuan.