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NCERT Solutions Class 10 Mathematics Solutions for Some Applications of Trigonometry - Exercise 9.1 in Chapter 9 - Some Applications of Trigonometry

Question 2 Some Applications of Trigonometry - Exercise 9.1

A 1.5 m tall boy is standing at some distance from a 30 m tall building. The angle of elevation from his eyes to the top of the building increases from 30° to 60° as he walks towards the building. Find the distance he walked towards the building.

Answer:

Let the boy initially stand at point Y with inclination 30° and then he approaches the building to the point X with inclination 60°.

NCERT Solutions - Mathematics , Class 10 chapter Some Applications of Trigonometry Question 2 Solution image

To Find: The distance boy walked towards the building i.e. XY

From figure,

XY = CD.

Height of the building = AZ = 30 m.

AB = AZ – BZ = 30 – 1.5 = 28.5

Measure of AB is 28.5 m

In right ΔABD,

tan 30° = AB/BD

1/√3 = 28.5/BD

BD = 28.5√3 m

Again,

In right ΔABC,

tan 60° = AB/BC

√3 = 28.5/BC

BC = 28.5/√3 = 28.5√3/3

Therefore, the length of BC is 28.5√3/3 m.

XY = CD = BD – BC = (28.5√3-28.5√3/3) = 28.5√3(1-1/3) = 28.5√3 × 2/3 = 57/√3 m.

Thus, the distance boy walked towards the building is 57/√3 m.

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