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Miscellaneous 3 · Q183

Q.Choose the correct option: ∫sin⁡(log⁡x) dx=\int \sin(\log x)\,dx =
(A) x2[sin⁡(log⁡x)−cos⁡(log⁡x)]+c\frac{x}{2}[\sin(\log x)-\cos(\log x)]+c (B) x2[sin⁡(log⁡x)+cos⁡(log⁡x)]+c\frac{x}{2}[\sin(\log x)+\cos(\log x)]+c (C) x2[cos⁡(log⁡x)−sin⁡(log⁡x)]+c\frac{x}{2}[\cos(\log x)-\sin(\log x)]+c (D) x4[cos⁡(log⁡x)−sin⁡(log⁡x)]+c\frac{x}{4}[\cos(\log x)-\sin(\log x)]+c

Maharashtra MsbshseTextbookSubjectiveImportance★★★★★
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Let I=∫sin⁡(log⁡x)dxI=\int\sin(\log x)dx. By parts with u=sin⁡(log⁡x)u=\sin(\log x), dv=dxdv=dx: I=xsin⁡(log⁡x)−∫x⋅cos⁡(log⁡x)⋅1x dx=xsin⁡(log⁡x)−∫cos⁡(log⁡x)dxI=x\sin(\log x)-\int x\cdot\cos(\log x)\cdot\frac1x\,dx=x\sin(\log x)-\int\cos(\log x)dx. By parts again on ∫cos⁡(log⁡x)dx\int\cos(\log x)dx with u=cos⁡(log⁡x)u=\cos(\log x): =xcos⁡(log⁡x)+∫sin⁡(log⁡x)dx=xcos⁡(log⁡x)+I=x\cos(\log x)+\int\sin(\log x)dx=x\cos(\log x)+I. So I=xsin⁡(log⁡x)−xcos⁡(log⁡x)−II=x\sin(\log x)-x\cos(\log x)-I, giving 2I=x[sin⁡(log⁡x)−cos⁡(log⁡x)]2I=x[\sin(\log x)-\cos(\log x)], so I=x2[sin⁡(log⁡x)−cos⁡(log⁡x)]+cI=\frac{x}{2}[\sin(\log x)-\cos(\log x)]+c. Checking: differentiating gives $\frac12[\sin(\ …

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