Example 1:
Air is being pumped into a spherical balloon at the rate of 50 cc per second. How fast is the radius of the balloon increasing when the radius is 15 cm? How fast is the radius increasing when the radius is 20 cm?

Problem 10 (Stewart, section 3.9):
A spotlight on the ground shines on a wall 12 m away. If a man 2 m tall walks from the spotlight toward the building at a speed of 1.6 m/s, how fast is his shadow on the building decreasing when he is 4 m from the building?

Problem 22 (Stewart, section 3.9):
A kite 100 ft above the ground moves horizontally at a speed of 8 ft/s. At what rate is the angle between the string and the horizontal decreasing when 200 ft of string have been let out?