Class CBSE Class 12 Mathematics Integrals Q #898
KNOWLEDGE BASED
APPLY
3 Marks 2023 SA
27. (a) Evaluate: $\int_{\pi/4}^{\pi/2}e^{2x}(\frac{1-\sin 2x}{1-\cos 2x})dx$

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Step-by-Step Solution

**Step 1: Simplify the integrand using trigonometric identities** We have: $\frac{1 - \sin 2x}{1 - \cos 2x} = \frac{1 - 2\sin x \cos x}{2\sin^2 x} = \frac{1}{2\sin^2 x} - \frac{2\sin x \cos x}{2\sin^2 x} = \frac{1}{2} \csc^2 x - \cot x$
**Step 2: Rewrite the integral** $\int_{\pi/4}^{\pi/2} e^{2x} (\frac{1 - \sin 2x}{1 - \cos 2x}) dx = \int_{\pi/4}^{\pi/2} e^{2x} (\frac{1}{2} \csc^2 x - \cot x) dx$
**Step 3: Manipulate the integrand to match the form $\int e^{ax} [f(x) + f'(x)] dx = e^{ax} f(x) + C$** We want to express the integrand in the form $e^{2x} [f(x) + f'(x)]$. Let's try to rewrite the integral as: $\int_{\pi/4}^{\pi/2} e^{2x} (-\cot x + \frac{1}{2} \csc^2 x) dx = -\frac{1}{2} \int_{\pi/4}^{\pi/2} e^{2x} (2\cot x - \csc^2 x) dx$ Now, consider $f(x) = \cot x$. Then $f'(x) = -\csc^2 x$. We need to adjust the integral to fit the form $\int e^{ax} [f(x) + \frac{1}{a}f'(x)] dx = e^{ax} \frac{f(x)}{a} + C$. Let's rewrite the integral as: $\int_{\pi/4}^{\pi/2} e^{2x} (-\cot x + \frac{1}{2} \csc^2 x) dx = \int_{\pi/4}^{\pi/2} e^{2x} (-\cot x + \frac{1}{2} \csc^2 x) dx$ We can rewrite this as: $\int_{\pi/4}^{\pi/2} e^{2x} (-\cot x + \frac{1}{2} \csc^2 x) dx = \int_{\pi/4}^{\pi/2} e^{2x} (-\cot x + \frac{1}{2} (-\cot x)') dx$ Using the formula $\int e^{ax} [f(x) + \frac{1}{a}f'(x)] dx = \frac{e^{ax}f(x)}{a} + C$, we have: $\int_{\pi/4}^{\pi/2} e^{2x} (-\cot x + \frac{1}{2} \csc^2 x) dx = \left[ e^{2x} \frac{-\cot x}{2} \right]_{\pi/4}^{\pi/2}$
**Step 4: Evaluate the definite integral** $\left[ -\frac{1}{2} e^{2x} \cot x \right]_{\pi/4}^{\pi/2} = -\frac{1}{2} \left[ e^{2(\pi/2)} \cot(\pi/2) - e^{2(\pi/4)} \cot(\pi/4) \right] = -\frac{1}{2} \left[ e^{\pi} (0) - e^{\pi/2} (1) \right] = \frac{1}{2} e^{\pi/2}$

Correct Answer: $\frac{1}{2}e^{\pi/2}$

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Pedagogical Audit
Bloom's Analysis: This is an APPLY question because it requires students to apply their knowledge of integration techniques, trigonometric identities, and exponential functions to solve the given definite integral.
Knowledge Dimension: PROCEDURAL
Justification: The question requires the student to execute a series of steps, including simplifying the integrand using trigonometric identities and then applying appropriate integration rules.
Syllabus Audit: In the context of CBSE Class 12, this is classified as KNOWLEDGE. The question directly tests the student's ability to apply standard integration techniques and trigonometric identities, which are core concepts covered in the textbook.