maths
posted by number theory .
When buying Brilliant Tshirts, Bradan determines that the total cost of buying N shirts is given by the function
C(N)=0.04N2−36N+9000
If he wants to minimize the total cost of buying shirts, how many shirts should he buy?

I will assume you meant:
C(n) = .04n^2  36n + 9000
C ' (n) = .08n  36
= 0 for a min of C
.08n = 36
n = 450