posted by Rob .
The daily sales, S, in thousands of dollars, is a function of the money, x, spent on advertising (in thousands of dollars) according to: S(x)= -x^3+9x^2+6/6. Find the x-coordinate of the point of diminishing returns. How much money is spent on advertising when you reach the point of diminishing returns?
Wouln't the point at which S'=0 change the sign of the returns?