Let the length of shadow be DE= y m and let the distance between the man and the lamp post be x meter.
Let AB be the height of lamp post and CD be the height of man.
∴ AB = 6m & CD = 2m
Therefore, by similar triangles theorem,
Differentiating eqn(1) w.r.t. time, t, we have
The rate at which the length of his shadow increases is 3km/hr