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Miscellaneous 3 · Q202

Q.Integrate: ∫cot⁡−1(1−x+x2) dx\int \cot^{-1}(1-x+x^2)\,dx

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Write cot⁡−1(1−x+x2)=tan⁡−1(11−x+x2)\cot^{-1}(1-x+x^2)=\tan^{-1}\left(\frac{1}{1-x+x^2}\right). Since 1+x(x−1)=1−x+x21+x(x-1)=1-x+x^2 and x−(x−1)=1x-(x-1)=1, this is tan⁡−1(x−(x−1)1+x(x−1))=tan⁡−1x−tan⁡−1(x−1)\tan^{-1}\left(\frac{x-(x-1)}{1+x(x-1)}\right)=\tan^{-1}x-\tan^{-1}(x-1) (tan-subtraction identity, valid on the relevant domain). Integrate each term by parts: ∫tan⁡−1x dx=xtan⁡−1x−12log⁡(1+x2)+c\int\tan^{-1}x\,dx=x\tan^{-1}x-\frac12\log(1+x^2)+c, and with w=x−1w=x-1, ∫tan⁡−1(x−1)dx=(x−1)tan⁡−1(x−1)−12log⁡(1+(x−1)2)+c\int\tan^{-1}(x-1)dx=(x-1)\tan^{-1}(x-1)-\frac12\log(1+(x-1)^2)+c. Subtracting: $x\tan^{-1}x-(x-1)\tan^{-1}(x-1)-\frac12\log(1+x^2)+\frac12\log(1+( …

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