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Miscellaneous 3 · Q205

Q.Integrate: ∫[log⁡(1+cos⁡x)−xtan⁡x2]dx\int \left[\log(1+\cos x)-x\tan\frac{x}{2}\right]dx

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Write 1+cos⁡x=2cos⁡2x21+\cos x=2\cos^2\frac x2, so log⁡(1+cos⁡x)=log⁡2+2log⁡cos⁡x2\log(1+\cos x)=\log2+2\log\cos\frac x2; thus ∫log⁡(1+cos⁡x)dx=xlog⁡2+2∫log⁡cos⁡x2 dx\int\log(1+\cos x)dx=x\log2+2\int\log\cos\frac x2\,dx. For the second piece, by parts with u=xu=x, dv=tan⁡x2 dxdv=\tan\frac x2\,dx (so v=∫tan⁡x2dx=−2log⁡cos⁡x2v=\int\tan\frac x2dx=-2\log\cos\frac x2): ∫xtan⁡x2 dx=−2xlog⁡cos⁡x2+2∫log⁡cos⁡x2 dx\int x\tan\frac x2\,dx=-2x\log\cos\frac x2+2\int\log\cos\frac x2\,dx. Subtracting, the two ∫log⁡cos⁡x2 dx\int\log\cos\frac x2\,dx terms cancel exactly: ∫[log⁡(1+cos⁡x)−xtan⁡x2]dx=xlog⁡2+2xlog⁡cos⁡x2+c=x[log⁡2+2log⁡cos⁡x2]+c=xlog⁡(2cos⁡2x2)+c=xlog⁡(1+cos⁡x)+c\int\left[\log(1+\cos x)-x\tan\frac x2\right]dx=x\log2+2x\log\cos\frac x2+c=x\left[\log2+2\log\cos\frac x2\right]+c=x\log\left(2\cos^2\frac x2\right)+c=x\log(1+\cos x)+c. Verified by di …

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