if the airplanes are flying at the same altitude, how fast will they be separating at 1:15 P.M.?

Question

An airplane flying north at 640 miles per hour passes over a certain town at noon. A second airplane going east at 400 miles per hour is directly over the same town 15 minutes later. if the airplanes are flying at the same altitude, how fast will they be separating at 1:15 P.M.? 

_______ miles per hour

Answer 1

Distance L(t) = [(640t)² + (400(t - 1/4))²]^1/2; dL/dt = [640t + 400(t - 1/4)]/L;

At t = 5/4 h dL/dt = (800 + 400)/(894.4272) mi/h = 1.342 mi/h;