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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 3.6 M Above the Ground, Find the Length of Her Shadow After 4 Seconds. - Mathematics

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 3.6 m above the ground, find the length of her shadow after 4 seconds.

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We have,

Height of girl = 90 cm = 0.9 m

Height of lamp-post = 3.6 m

Speed of girl = 1.2 m/sec

∴ Distance moved by girl (CQ) = Speed × Time

= 1.2 × 4 = 4.8m

Let length of shadow (AC) = x cm


∠ACB = ∠AQP                          [Each 90°]

∠BAC = ∠PAQ                         [Common]

Then, ΔAB ~ ΔAPQ                  [By AA similarity]

`therefore"AC"/"AQ"="BC"/"PQ"`                 [Corresponding parts of similar Δ are proportional]



⇒ 4x = x + 4.8

⇒ 4x – x = 4.8

⇒ 3x = 4.8

`rArrx=4.8/3=1.6` m

∴ Length of shadow = 1.6m

Concept: Triangles Examples and Solutions
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RD Sharma Class 10 Maths
Chapter 7 Triangles
Exercise 7.5 | Q 21 | Page 75
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