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  • Explain solution RD Sharma class 12 chapter 27 Straight Line in Space exercise Multiple choice question 8 maths

Explain solution RD Sharma class 12 chapter 27 Straight Line in Space exercise Multiple choice question 8 maths

Answers (1)

Answer: Correct answer is B.

Hint: Use trigonometric formulae

Given: \cos 2 \alpha+\cos 2 \beta+\cos 2 \gamma

Solution: \cos 2 \alpha+\cos 2 \beta+\cos 2 \gamma=2\left(\cos ^{2} \alpha+\cos ^{2} \beta+\cos ^{2} \gamma\right)-3

using \left(\cos 2 \theta=2 \cos ^{2} \theta-1\right)

And as the question states: A line makes angle \alpha ,\beta, \gamma with the axes respectively

\begin{aligned} &\text { So, } \cos 2 \alpha+\cos 2 \beta+\cos 2 \gamma=2\left(\cos ^{2} \alpha+\cos ^{2} \beta+\cos ^{2} \gamma\right)-3 \\\\ &=2-3 \\\\ &=-1 \end{aligned}

So, the correct option is (b)-1

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infoexpert26

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