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Exercise 11.7 · Q2

Q.Integrate the following with respect to xx:

(i) xlog⁡xx\log x
(ii) 27x2e3x27x^{2}e^{3x}
(iii) x2cos⁡xx^{2}\cos x
(iv) x3sin⁡xx^{3}\sin x
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log⁡x\log x has no direct formula so it must be uu (rule (i) of §11.7.5); the other three have u=xnu=x^n for n≥2n\ge2, so Bernoulli's formula (§11.7.6) is the efficient route.

Part (i): xlog⁡xx\log x. Take u=log⁡x, dv=x dx⇒du=1xdx, v=x22u=\log x,\,dv=x\,dx\Rightarrow du=\dfrac1x dx,\,v=\dfrac{x^2}2.

∫xlog⁡x dx=x2log⁡x2−∫x22⋅1x dx=x2log⁡x2−12∫x dx=x2log⁡x2−x24+c\displaystyle\int x\log x\,dx=\dfrac{x^2\log x}2-\int\dfrac{x^2}2\cdot\dfrac1x\,dx=\dfrac{x^2\log x}2-\dfrac12\int x\,dx=\dfrac{x^2\log x}2-\dfrac{x^2}4+c.

Check: ddx[x2log⁡x2−x24]=xlog⁡x+x2−x2=xlog⁡x\dfrac d{dx}\left[\dfrac{x^2\log x}2-\dfrac{x^2}4\right]=x\log x+\dfrac x2-\dfrac x2=x\log x ✓.

Part (ii): 27x2e3x27x^2e^{3x}. Bernoulli with u=x2 (u′=2x, u′′=2)u=x^2\,(u'=2x,\,u''=2), dv=e3xdx (v=e3x3, v1=e3x9, v2=e3x27)dv=e^{3x}dx\,(v=\tfrac{e^{3x}}3,\,v_1=\tfrac{e^{3x}}9,\,v_2=\tfrac{e^{3x}}{27}):

∫x2e3xdx=x2⋅e3x3−2x⋅e3x9+2⋅e3x27+c=x2e3x3−2xe3x9+2e3x27+c\displaystyle\int x^2e^{3x}dx=x^2\cdot\dfrac{e^{3x}}3-2x\cdot\dfrac{e^{3x}}9+2\cdot\dfrac{e^{3x}}{27}+c=\dfrac{x^2e^{3x}}3-\dfrac{2xe^{3x}}9+\dfrac{2e^{3x}}{27}+c.

Multiplying by 2727: 27∫x2e3xdx=9x2e3x−6xe3x+2e3x+c27\displaystyle\int x^2e^{3x}dx=9x^2e^{3x}-6xe^{3x}+2e^{3x}+c.

Check: ddx[9x2e3x−6xe3x+2e3x]=18xe3x+27x2e3x−6e3x−18xe3x+6e3x=27x2e3x\dfrac d{dx}[9x^2e^{3x}-6xe^{3x}+2e^{3x}]=18xe^{3x}+27x^2e^{3x}-6e^{3x}-18xe^{3x}+6e^{3x}=27x^2e^{3x} ✓.

Part (iii): x2cos⁡xx^2\cos x. Bernoulli with u=x2 (u′=2x, u′′=2)u=x^2\,(u'=2x,\,u''=2), dv=cos⁡x dx (v=sin⁡x, v1=−cos⁡x, v2=−sin⁡x)dv=\cos x\,dx\,(v=\sin x,\,v_1=-\cos x,\,v_2=-\sin x):

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

Check: ddx[x2sin⁡x+2xcos⁡x−2sin⁡x]=2xsin⁡x+x2cos⁡x+2cos⁡x−2xsin⁡x−2cos⁡x=x2cos⁡x\dfrac d{dx}[x^2\sin x+2x\cos x-2\sin x]=2x\sin x+x^2\cos x+2\cos x-2x\sin x-2\cos x=x^2\cos x ✓. …

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