A 10-ft ladder is leaning against a house when its base starts to slide away. By the time the base is 8ft from the house, the base is moving away at the rate of 18ft/sec. a. What is the rate of change of the height of the top of the ladder? 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?