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A girl of height 100 cm …

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A girl of height 100 cm is walking away from the Base of a lamppost at a speed of 1.9m/s.If the lamp is 5m above the ground ,find the length of her shadow after 45 seconds.
  • 1 answers

Sia ? 5 years, 11 months ago


Let AB be the lamp-post, DE be the girl and D be the position of girl after 4s.
Again, let DC = xm be the length of shadow of the girl.
Given, DE = 100 cm =1 m, AB =5 m and speed of the girl= 1.9 m/s
Distance of the girl from lamp-post after 4 s = BD = 1.9×4=7.6m [ Distance = speed × time ]
In ABC and EDCB=D
[each 90o
C=C [ common angle ] 
ABCEDC [ by AA similarity criterion ]
BCDC=ABDE [since, corresponding sides of similar triangles are proportional]...(i)
On substituting all the values in Eq(i), we get 
7.6+xx=51 
7.6+x=5x
7.6=5xx
7.6=4x
x=7.64 
x=1.9m
Hence the length of her shadow after 4 s is 1.9 m.

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