Question

A plane flying horizontally at an altitude of 2 km and a speed of 600 km/h passes directly over a radar station. Find the rate (in km/h) at which the distance from the plane to the station is increasing when it is 3 km away from the station. (Round your answer to the nearest whole number.)

Answer 1

When the plane is 2mi away from the radar station, its distance's increase rate is approximately 433mi/h.

Explanation:

Sketch a right triangle with one leg going vertically a length of 3 miles above the station, and the other leg x going horizontally from the top of the first leg. Distance r from plane to station is related by r2 = x2 + 32. Differentiate: 2r dr/dt = 2x dx/dt, which rearranges to the desired dr/dt = x/r dx/dt. When r = 4, x2 = 42 - 9 and x = √7. So dr/dt = (√7 / 4) * 520 mph = 344 mph.