Question

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

Answer 1

length of shadow after 4 sec will be 1.6 m long

Answer 2

Let AB be lamp post, PQ be the girl and QR be her shadow.

Speed of girl = 1.2m/s
So distance travelled in 4 second = 1.2×4 =4.8m

So BQ = 4.8m

Now As PQ||AB so ∆RPQ ~ ∆RAB
So corresponding sides are proportionate.

$\frac{PQ}{AB}$ = $\frac{RQ}{BR}$

$\frac{0.9}{4.8}$ = $\frac{RQ}{RQ+4.8}$

$\frac{3}{16}$ = $\frac{RQ}{RQ+4.8}$

3RQ + 14.4 = 16RQ
13RQ = 14.4
RQ = 1.103