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Q.Solve: ∫x2sin⁡x dx\displaystyle\int x^2 \sin x\, dx

Maharashtra MsbshseMaharashtra HSC (MSBSHSE) Board 2020Subjective· 4mImportance★★★★★
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By parts with u=x2u = x^2: ∫x2sin⁡x dx=−x2cos⁡x+2∫xcos⁡x dx\int x^2\sin x\,dx = -x^2\cos x + 2\int x\cos x\,dx; a second by-parts gives ∫xcos⁡x dx=xsin⁡x+cos⁡x\int x\cos x\,dx = x\sin x + \cos x, so the answer is −x2cos⁡x+2xsin⁡x+2cos⁡x+c-x^2\cos x + 2x\sin x + 2\cos x + c.

Use integration by parts ∫u dv=uv−∫v du\int u\,dv = uv - \int v\,du with u=x2u = x^2, dv=sin⁡x dxdv = \sin x\,dx (so du=2x dxdu = 2x\,dx, v=−cos⁡xv = -\cos x):

∫x2sin⁡x dx=x2(−cos⁡x)−∫(−cos⁡x)(2x) dx=−x2cos⁡x+2∫xcos⁡x dx.\int x^2 \sin x\,dx = x^2(-\cos x) - \int (-\cos x)(2x)\,dx = -x^2\cos x + 2\int x\cos x\,dx.

Now integrate ∫xcos⁡x dx\int x\cos x\,dx by parts with u=xu = x, dv=cos⁡x dxdv = \cos x\,dx (du=dxdu = dx, v=sin⁡xv = \sin x): …

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