An observer is 21 m above the ground floor of a large hotel atrium looking at a glass-enclosed elevator shaft that is 15 m horizontally from the observer (see figure). The angle of elevation of the elevator is the angle that the observer's line of sight makes with the horizontal (it may be positive or negative). Assuming that the elevator rises at a rate of 6 m/s, what is the rate of change of the angle of elevation when the elevator is 16 m above the ground? When the elevator is 26 m above the ground?