8,9With differentiation rules thx

8. If a snowball melts so that its surface area decreases at a rate of 1 cm / min, find the rate at which the diameter decreases when the diameter is 10 cm. 9. A street light is mounted at the top of a 15-ft-tall pole. A man 6 ft tall walks away from the pole with a speed of 5 ft/s along a straight path. How fast is the tip of his shadow moving when he is 40 ft from the pole