Question: There are two vertical posts, one on each side of a road, just opposite to each other. One post is 108 m high. From the top of this post, the angles of depression of the top and foot of the other post are $ 30{}^\circ $ and $ 60{}^\circ , $ respectively. The height of the other post, (in metre) is
Options:
A) 36
B) 72
C) 108
D) 110
Show Answer
Answer:
Correct Answer: B
Solution:
- Let the height of other part be h m and distance between the post be x m.
In $ \Delta ABC, $
$ \tan 60{}^\circ =\frac{AB}{BC} $
$ \Rightarrow $ $ \sqrt{3}=\frac{108}{BC} $
$ \Rightarrow $ $ BC=\frac{108}{\sqrt{3}}=36\sqrt{3}m $
In $ \Delta AED, $
$ \tan 30{}^\circ =\frac{AE}{ED} $ $ [\because ED=BC] $
$ \Rightarrow $ $ \frac{1}{\sqrt{3}}=\frac{108-h}{36\sqrt{3}} $
$ \Rightarrow $ $ 108-h=36 $
$ \Rightarrow $ $ h=108-36 $
$ \Rightarrow $ $ h=72m $