JEE Main & Advanced Mathematics Trigonometric Identities Question Bank Trigonometrical ratios of sum and difference of two and three angles

  • question_answer
    If \[\tan x+\tan \left( \frac{\pi }{3}+x \right)+\tan \left( \frac{2\pi }{3}+x \right)=3,\] then

    A) \[\tan x=1\]

    B) \[\tan 2x=1\]

    C) \[\tan 3x=1\]

    D) None of these

    Correct Answer: C

    Solution :

    \[\tan x+\tan \,\left( \frac{\pi }{3}+x \right)+\tan \,\left( \frac{2\pi }{3}+x \right)\] \[=\tan x+\frac{\tan x+\sqrt{3}}{1-\sqrt{3}\,\tan x}+\frac{\tan x-\sqrt{3}}{1+\sqrt{3}\,\tan x}\] \[=\tan x+\frac{8\tan x}{1-3{{\tan }^{2}}x}=\frac{3\,(3\tan x-{{\tan }^{3}}x)}{1-3{{\tan }^{2}}x}=3\tan 3x\] Therefore, the given equation is \[3\tan 3x=3\] Þ \[\tan 3x=1.\]


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