Porisma

Worked solution

Complete a square after substitution and use a product derivative

(a) By using the substitution u=xu=\sqrt x, find ∫1x (x+2x+2) dx\displaystyle\int\dfrac1{\sqrt x\,(x+2\sqrt x+2)}\,dx. marks

(b) Find ddx(xsin⁡x)\dfrac d{dx}(x\sin x). mark

Hence find ∫(sin⁡x+xcos⁡x)ln⁡x dx\displaystyle\int(\sin x+x\cos x)\ln x\,dx. marks

[2016/PJC/Promo/7]

CJC · Review Problems · Question 8

Answer

(a) 2tan⁡−1(x+1)+C2\tan^{-1}(\sqrt x+1)+C; (b) xcos⁡x+sin⁡xx\cos x+\sin x and xsin⁡xln⁡x+cos⁡x+Cx\sin x\ln x+\cos x+C.

(a)

Let u=xu=\sqrt x, so dx=2u dudx=2u\,du. The integral becomes ∫2u2+2u+2du=∫2(u+1)2+1du=2tan⁡−1(u+1)+C\int\dfrac2{u^2+2u+2}du=\int\dfrac2{(u+1)^2+1}du=2\tan^{-1}(u+1)+C. Thus the answer is 2tan⁡−1(x+1)+C2\tan^{-1}(\sqrt x+1)+C for x>0x>0.

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(b) Derivative

The product rule gives ddx(xsin⁡x)=xcos⁡x+sin⁡x\dfrac d{dx}(x\sin x)=x\cos x+\sin x.

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(b) Hence integral

Use u=ln⁡xu=\ln x and dv=(sin⁡x+xcos⁡x)dxdv=(\sin x+x\cos x)dx, so du=dx/xdu=dx/x and the preceding derivative gives v=xsin⁡xv=x\sin x. The integral is xsin⁡xln⁡x−∫sin⁡x dx=xsin⁡xln⁡x+cos⁡x+Cx\sin x\ln x-\int\sin x\,dx=x\sin x\ln x+\cos x+C.

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Common mistakes

The log in the hence integral restricts x to positive inputs.

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