Chapter 9.7, Problem 43E

Advertising and sales An inferior product with an extensive advertising campaign does well when it is released, but sales decline as people discontinue use of the product. If the sales S (in thousands of dollars) after t weeks are given by

$S\left(t\right)=\frac{200t}{{\left(t+1\right)}^2},t\le 0$ what is the rate of change of sales when t = 9? Interpret your result.