a girl of height 90cm is walking away from the base of a lamp post at a speed of 1.2m/s if the lamps is 3.6 m above the ground, find

If your question is:

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

Then the solution is:

Let the girl be at point D on the ground from the lamppost after 4 sec.
∴ AD = 1.2 m/sec × 4 sec = 4.8 m = 480 cm
Suppose the length of the shadow of the girl be x cm when she at position D.
∴ CD = x cm
In ∆CDE and ∆CAB
∠CDE = ∠CAB  (90°)
∠DCE = ∠ACB  (Common)
∴ ∆CDE ∼ ∆CAB  (AA similarity)

 

∴ Length of her shadow after 4 second is 160 cm

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