Solution :
Given :
The height of the person is 5 foot.
The height of the lamppost is 12 foot.
Speed of the person away from the lamppost = 4 ft/s
Let the distance of the tip of the shadow of the perosn to the pole at t seconds after the person started walking away from the pole be d(t).
By similarity of the triangles, we see that
.................(i)
Here x is the distance from the person to the tip of the shadow. As it is given the speed of the person is 4 ft/s, then the distance of the person from the pole 4t. So we have,
x = d - 4t .............(ii)
Putting (ii) in (i) and solving for d is
d(t) = 6.85 t
So now if we derive d(t), we will get the wanted rate of the tip of the shadow.
∴ d'(t) = 6.85 ft/s
