An aircraft is flying at a constant altitude with a constant speed of 600 mi/hr. An antiaircraft missile is fire on a straight line to the flight path of the aircraft so that it will hit the aircraft at point P. Suppose that the angle between the flight paths from point P is 120 degrees. At the instant the aircraft is 2 miles from the impact point P, the missile is 4 miles from P and flying at 1200 mi/hr. At that instant, how rapidly is the distance between the missile and the aircraft decreasing?
I think that I need to use the law of cosines.