A 16-ft ladder is leaning against a house when its base starts to slide away. By the time the base is 13 ft from the house, the base is moving at the rate of 5 ft/sec. a. How fast is the top of the ladder sliding down the wall then? b. At what rate is the area of the triangle formed by the ladder, wall, and ground changing then? c. At what rate is the angle between the ladder and the ground changing then?