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Q.Differentiate tan⁡−1(3x−x31−3x2)\tan^{-1}\left(\dfrac{3x-x^{3}}{1-3x^{2}}\right) with respect to xx.

Nagaland NbseNagaland Board of School Education 2018Subjective· 2mImportance★★★★★
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Recognise (3x−x3)/(1−3x2)(3x-x^3)/(1-3x^2) as the tan triple-angle identity to collapse the expression to 3tan⁡−1x3\tan^{-1}x.

Recall the identity tan⁡3θ=3tan⁡θ−tan⁡3θ1−3tan⁡2θ\tan3\theta = \dfrac{3\tan\theta-\tan^3\theta}{1-3\tan^2\theta}. Put x=tan⁡θx=\tan\theta, so θ=tan⁡−1x\theta=\tan^{-1}x:

3x−x31−3x2=tan⁡3θ\dfrac{3x-x^3}{1-3x^2} = \tan3\theta …

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