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For $$\alpha, \beta \in(0, \pi / 2)$$, let $$3 \sin (\alpha+\beta)=2 \sin (\alpha-\beta)$$ and a real number $$k$$ be such that $$\tan \alpha=k \tan \beta$$. Then, the value of $$k$$ is equal to

JEE · Math · previous-year question

  1. A.5
  2. B.$$-$$2/3
  3. C.$$-$$5correct
  4. D.2/3

Answer

C. $$-$$5

Explanation

To find the value of $$k$$, the given conditions are: $$3 \sin (\alpha+\beta)=2 \sin (\alpha-\beta)$$ And $$\tan \alpha = k \tan \beta$$ For the first equation, using the sum and difference formulas for sine, we can rewrite the equation as: $$3(\sin \alpha \cos \beta + \cos \alpha \sin \beta) = 2(\sin \alpha \cos \beta - \cos \alpha \sin \beta)$$ Simplifying this, we get: $$3 \sin \alpha \cos \beta + 3 \cos \alpha \sin \beta = 2 \sin \alpha \cos \beta - 2 \cos \alpha \sin \beta$$ Rearranging the terms, we obtain: $$5 \sin \beta \cos \alpha = - \sin \alpha \cos \beta$$ Dividing both sides by $$\sin \alpha \cos \beta$$, we get: $$\frac{5 \sin \beta \cos \alpha}{\sin \alpha \cos \beta} = -1$$ Which simplifies to: $$5 \tan \beta = - \tan \alpha$$ So, taking the reciprocal, we have: $$\tan \alpha = -5 \tan \beta$$ Therefore, by comparing this equation with the given $$\tan \alpha = k \tan \beta$$, we find that $$k = -5$$. Thus, the value of $$k$$ is $$-5$$.

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