400
A 13-ft ladder is leaning against a house when the base starts to slide away. By the time the base is 12 ft from the house, the base is moving at the rate of 5 ft/sec and the height is moving at the rate of 3 ft/sec.
At what rate is the area of the triangle (formed by the ladder, house and ground) changing?
da/dt = 30.5 square feet per second