I have this problem, and I have attepmted a classical approach without much success. A man 5 ft tall runs at a rate of 8ft/sec towards a source of light that arises vertically at a point A. The height of the light source H, is given by the formula h(t)=t^3 +1, in feet, where time t is measured in seconds. At what rate is the length of the man's shadow decreasing at time=10seconds? I have tried establishing the geometrical relationships between the height h(t) and the distance between the point A and the tip of the man's shadow. say, if we call "s" the length of the shadow, and 'x' the distance between the man and point A, we have that h/5=x/s but then I do not have any other information about the problem. What's next??