A girl of height 90 cm is walking away from the base of a lamp-post at a speed of 1.2 m/s. If the lamp is 3.6 m above the ground, find the length of her shadow after 4 seconds.

Dear student,

Take a right angled triangle and use pythagoras theorem

a2+v2=l2

Now differentaite both sides and da/dt=0 since length of wall is constant and is not changing with time....