Here we use similar triangles. The height of the lamp is to the end of the shadow, the length of the shadow will be (d+l) as the woman's height is 5 ft, the length of the woman's shadow will be l.
Thus we shall have the following similarity
(height of the SL)/(length of the shadow of SL)=(height of the woman)/(length of her shadow)
hence
14/(d+l)=5/l
cross multiplying we get
14l=5(d+l)
14l=5d+5l
9l=5d
l=5/9d
the function can be written as
l(d)=5/9d
