Class 10 chapter 9 exercise number 6
Answers
Answer:
6. 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.
Solution:
Let the boy initially stand at point Y with inclination 30° and then he approaches the building to the point X with inclination 60°.
C:\Users\User\Desktop\NCERT\images\trig6.jpg
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 = 19√3 m.
Thus, the distance boy walked towards the building is 19√3 m.
Explanation: