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Q.If y=tan⁡−1x2−cot⁡−1x2y = \tan^{-1}\dfrac{x}{2} - \cot^{-1}\dfrac{x}{2}, then dydx\dfrac{dy}{dx} is

(a) 44+x2\dfrac{4}{4+x^{2}}
(b) 24+x2\dfrac{2}{4+x^{2}}
(c) 14+x2\dfrac{1}{4+x^{2}}
(d) 21+x2\dfrac{2}{1+x^{2}}
Nagaland NbseNagaland Board of School Education 2017MCQ· 1mImportance★★★★★
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Use cot⁡−1θ=π2−tan⁡−1θ\cot^{-1}\theta=\frac\pi2-\tan^{-1}\theta to rewrite y purely in terms of tan⁡−1(x/2)\tan^{-1}(x/2), then differentiate.

y=tan⁡−1x2−cot⁡−1x2y = \tan^{-1}\dfrac x2 - \cot^{-1}\dfrac x2. Since cot⁡−1x2=π2−tan⁡−1x2\cot^{-1}\dfrac x2 = \dfrac\pi2-\tan^{-1}\dfrac x2:

y=tan⁡−1x2−(π2−tan⁡−1x2)=2tan⁡−1x2−π2y = \tan^{-1}\dfrac x2 - \left(\dfrac\pi2-\tan^{-1}\dfrac x2\right) = 2\tan^{-1}\dfrac x2 - \dfrac\pi2 …

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