Question : basic calculus - finding the maxium

Hi

If you have R = D^2(C/2  -  D/3)

Then isn't dR/dD = CD -D^2 so that you have max/min points when D=0 or D=C?

I must be making silly mistake as the book says the max point is when D = C/2

thanks

Answer : basic calculus - finding the maxium

R(x) = x^2(C/2  -  x/3) =  (C/2) *  x^2 -  x^3 /3

R'(x) =  (C/2) *2  x -  3 x^2 /3
R'(x) =  C x -  x^2 =   C x -  x^2 = x ( C -  x )
R'(x) = 0, when x = C and x = 0