Question

The length of a rectangle is increasing at a rate of 7 cm/s and its width is increasing at a rate of 3 cm/s. When the length is 7 cm and the width is 5 cm, how fast is the area of the rectangle increasing (in cm2/s)

Answer 1

A = lw

dA = l(dw) + w(dl)

= (7 cm)(7 cm/s) + (5 cm)(3 cm/s)

= (49 + 15) cm²/s = 64 cm²/s