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Q.Find ∫xtan⁡−1x dx\int x \tan^{-1} x\,dx.

Karnataka PUCKarnataka II PUC Board 2024Subjective· 3mImportance★★★★★
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Integration by parts (ILATE) gives ∫xtan⁡−1x dx=x2+12tan⁡−1x−x2+c\int x\tan^{-1}x\,dx=\dfrac{x^2+1}{2}\tan^{-1}x-\dfrac{x}{2}+c.

Step 1 — Choose parts (ILATE rule).

Take the inverse-trig factor as the first function and xx as the second:

u=tan⁡−1x,dv=x dx.u=\tan^{-1}x,\qquad dv=x\,dx.

Then

du=11+x2 dx,v=x22.du=\frac{1}{1+x^2}\,dx,\qquad v=\frac{x^2}{2}.

Step 2 — Apply ∫u dv=uv−∫v du\int u\,dv=uv-\int v\,du.

∫xtan⁡−1x dx=x22tan⁡−1x−∫x22⋅11+x2 dx=x22tan⁡−1x−12∫x21+x2 dx.\int x\tan^{-1}x\,dx=\frac{x^2}{2}\tan^{-1}x-\int\frac{x^2}{2}\cdot\frac{1}{1+x^2}\,dx=\frac{x^2}{2}\tan^{-1}x-\frac12\int\frac{x^2}{1+x^2}\,dx.

Step 3 — Simplify the remaining integrand.

x21+x2=(1+x2)−11+x2=1−11+x2.\frac{x^2}{1+x^2}=\frac{(1+x^2)-1}{1+x^2}=1-\frac{1}{1+x^2}.

Hence …

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