You are stationary on the ground and are watching a bird fly away horizontally at a rate of 10 meters per second. The bird is located 40 meters above your head. Let θ represent the angle between the vertical axis and your line of sight to the bird (this is sometimes called the angle of elevation). Find the rate of change of the angle of elevation as a function of time so that your eyes stay focused on the bird when the horizontal distance between you and the bird is 9 meters.