Therefore, we need to differentiate each side of our area equation with respect to t using the product rule. Doing so gives the following result where d(w) dA dt dt dA dt =0 Z 1 ) ON dw dt W W dt Step 3 We are given that the length is increasing at a rate of 4 cm/s and the width is increasing at a rate of 9 cm/s. Therefore, we have the following. dl = dt dw