Question

An aeroplane at an height of 1 km is flying horizontally at 800 km/hr passes directly over an observer .find the rate at which it is approaching the observer when it is 1250 m away from him

Answer 1

h = 1000
x = sqrt (1250^2 - 1000^2)
= 750 meters horizontal
r = distance (hypotenuse)

r^2 = h^2 + x^2
2 r dr/dt = 0 + 2 x dx/dt
so
dr/dt = (x/r) dx/dt
= (750/1250)(800)

Answer 2

Ans is 480 km per second..