Q10 of 56 Page 127

A give 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 at a height of 3.6 m above the ground, find the length of her shadow after 4 seconds.

The speed of give = 1.2 ms – 1


Time taken = 4 seconds


Distance = speed × time taken


In 4 seconds, the distance walked is 1.2×4 = 4.8 m.


As both the lamp post and the give are perpendicular to the ground, they make the following similar triangles.



In ΔBDE & ΔBAC,


∠B = ∠B |common angle


DE||AC


∠BDE = ∠BAC


ΔBDE~ΔBAC by AA Similarity Rule




4x = x + 4.8


⇒ x = 1.6 m


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