A balloon rising from the ground at 140 ftmin is tracked by

A balloon rising from the ground at 140 ft/min is tracked by a rangefinder at point A, located 500 ft from the point of liftoff. Find the rate at which the distance between the range finder and the balloon is changing when the balloon is 500 ft above the ground.

Solution

the angle at that point = arctan (500 / 500)
= 45 degree

so , the rate of going away = 141 X 1/ 2

= 99.70 ft/min