A man 1.8m tall walks away from a lamp post 4m high at a speed of 1.5 m/s. How fast is his shadow lengthen ?

Answer 1

When he is x m from the pole, and his shadow is s m long, we have
s/1.8 = (x+s)/4
ds/dt = 1/4 (dx/dt + ds/dt)
3 ds/dt = 1.5
ds/dt = 0.5 m/s