y =[70000+2000+2000 x+(2000+ 1000+ 1000 x)* x/2]/x

It is simplified as y = (72000+3500x+500x 2)/x, that is, y=72000/x+3500+500x.

In fact, when finding the minimum value of 72000/x+500x, what is x?

It should be noted that x must be greater than 0, which is the process and will be counted.

It should be the minimum value when 72000/x = 500 x.

That is, x 2 = 144 and x = 12.

12 years, with the lowest average cost.