Two trains begin their journey from the same station. The first train leaves at 12 noon and was traveling at 80 mph due south and the second train leaves the station an hour after heading east when a speed of 90 mph. How fast was the distance between the two trains changing at 8 PM? Use the derivative, (dy/dx), of a function, f(x) = y , is its instantaneous rate of change with respect to the variable x.