Question: The height of a tower is h and the angle of elevation of the top of the tower is $ \alpha . $ On moving a distance h/2 towards the tower, the angle of elevation becomes $ \beta . $ What is the value of $ (\cot \alpha -\cot \beta ) $ ?
Options:
A) $ \frac{1}{2} $
B) $ \frac{2}{3} $
C) 1
D) 2
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Answer:
Correct Answer: A
Solution:
- [a] $ \tan \beta =\frac{h}{CD} $
$ \Rightarrow $ $ CD=\frac{h}{\tan \beta }=h\cot \beta $ and $ \tan \alpha =\frac{h}{\frac{h}{2}+CD} $
$ \Rightarrow $ $ \frac{h}{2}+CD=\frac{h}{\tan \alpha }=h\cot \alpha $
$ \Rightarrow $ $ \frac{h}{2}+h\cot \beta =h\cot \alpha $
$ \Rightarrow $ $ h\cot \alpha -h\cos \beta =\frac{h}{2} $
$ \Rightarrow $ $ (\cot \alpha -\cot \beta )=\frac{h}{2} $
$ \therefore $ $ \cot \alpha -cot\beta =\frac{1}{2} $