math help~~~~~~~~~~~~?

Question

The length of a rectangle is increasing at a rate of 5 cm/s and its width is increasing at a rate of 4 cm/s. When the length is 10 cm and the width is 12 cm, how fast is the area of the rectangle increasing?

Answer 1

Area = A = w*l

dA/dt = w*(dl/dt) + l*(dw/dt)

dl/dt = 5 cm/s, dw/dt = 4 cm/s

When w = 12 cm & l = 10 cm ,

dA/at = (12 cm) * (5 cm/s) + (10 cm)*(4 cm/s)

= 100 cm^2/s