Question

How to do this and find the max or min points

Answer 1

the min is 101.1

Answer 2

I see your last line is : c(x) = 0.9(x^2-10)^2 + 101.1

Let y = x^2, then c(y) = 0.9(y-10)^2 + 101.1

Apparently, c(y) is a parabola, min is 101.1 when y = 10, max is infinity

So let x^2 = 10 -> x = sqrt(10) or -sqrt(10), min is 101.1, max is infinity