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Q.Evaluate : ∫π/2πex(1−sin⁡x1−cos⁡x)dx\int_{\pi/2}^{\pi} e^x \left(\frac{1-\sin x}{1-\cos x}\right) dx

CBSECBSE Class XII Board 2025Subjective· 3mImportance★★★★★
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Writing the integrand as ex(f(x)+f′(x))e^x\big(f(x)+f'(x)\big) with f(x)=−cot⁡x2f(x) = -\cot\frac{x}{2} gives ∫π/2πex 1−sin⁡x1−cos⁡x dx=eπ/2\displaystyle\int_{\pi/2}^{\pi} e^x\,\frac{1-\sin x}{1-\cos x}\,dx = e^{\pi/2}.

Simplify the integrand

Using 1−cos⁡x=2sin⁡2x21-\cos x = 2\sin^2\frac{x}{2} and sin⁡x=2sin⁡x2cos⁡x2\sin x = 2\sin\frac{x}{2}\cos\frac{x}{2}:

1−sin⁡x1−cos⁡x=1−2sin⁡x2cos⁡x22sin⁡2x2=12csc⁡2x2−cot⁡x2.\frac{1-\sin x}{1-\cos x} = \frac{1 - 2\sin\frac{x}{2}\cos\frac{x}{2}}{2\sin^2\frac{x}{2}} = \frac{1}{2}\csc^2\frac{x}{2} - \cot\frac{x}{2}.

Recognise the ex[f(x)+f′(x)]e^x[f(x)+f'(x)] form

Let f(x)=−cot⁡x2f(x) = -\cot\frac{x}{2}. Then f′(x)=12csc⁡2x2f'(x) = \frac{1}{2}\csc^2\frac{x}{2}, so

1−sin⁡x1−cos⁡x=f(x)+f′(x),\frac{1-\sin x}{1-\cos x} = f(x) + f'(x),

and using ∫ex[f(x)+f′(x)] dx=exf(x)+C\displaystyle\int e^x[f(x)+f'(x)]\,dx = e^x f(x) + C:

∫ex 1−sin⁡x1−cos⁡x dx=−excot⁡x2+C.\int e^x\,\frac{1-\sin x}{1-\cos x}\,dx = -e^x\cot\frac{x}{2} + C. …

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