related rate problem
posted by george .
A railroad track and a road cross at a right angle. An observer stands on the road 70 meters south of the crossing and watches an eastbound train traveling at 60 meters per second. At how many meters per second is the train moving away from the observer 4 seconds after it passes through the intersection?

Well, 4 seconds after crossing, the train has traveled 240 m. Draw your figure, lets are 60, 240, and the hypotenuse is L=sqrt (60^2+240^2)
But in general, l^2=x^2+60^2
2L*dL/dt=2x dx/dt
you are solveing for dL/dt, you know x, dx/dt, and L.