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Miscellaneous 3 · Q177

Q.Choose the correct option: ∫x−sin⁡x1−cos⁡xdx=\int \frac{x-\sin x}{1-\cos x}dx =
(A) xcot⁡x2+cx\cot\frac{x}{2}+c (B) −xcot⁡x2+c-x\cot\frac{x}{2}+c (C) cot⁡x2+c\cot\frac{x}{2}+c (D) xtan⁡x2+cx\tan\frac{x}{2}+c

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Split ∫x−sin⁡x1−cos⁡xdx=∫x1−cos⁡xdx−∫sin⁡x1−cos⁡xdx\int\frac{x-\sin x}{1-\cos x}dx=\int\frac{x}{1-\cos x}dx-\int\frac{\sin x}{1-\cos x}dx. For the second piece, let u=1−cos⁡xu=1-\cos x, du=sin⁡x dxdu=\sin x\,dx, giving log⁡(1−cos⁡x)\log(1-\cos x). For the first, use 1−cos⁡x=2sin⁡2x21-\cos x=2\sin^2\frac x2, so it is 12∫xcsc⁡2x2 dx\frac12\int x\csc^2\frac x2\,dx. By parts with u=xu=x, dv=csc⁡2x2 dxdv=\csc^2\frac x2\,dx (so v=−2cot⁡x2v=-2\cot\frac x2): 12[−2xcot⁡x2+2∫cot⁡x2 dx]=−xcot⁡x2+2log⁡∣sin⁡x2∣\frac12\left[-2x\cot\frac x2+2\int\cot\frac x2\,dx\right]=-x\cot\frac x2+2\log\left|\sin\frac x2\right|. Combining with $-\log(1-\cos x)=-\log\l …

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