Definite Integration
Advanced Integration Properties
Grade 12
Question:
<p>Suppose <span class="math">V = \int_{0}^{\pi/2} \frac{x \sin 2x}{2\sin^2 x + 1} dx</span>, find the value of <span class="math">96V</span>.</p>
Step-by-Step Solution
Key Concept: Recognize that this integral involves a product of a linear term and a trigonometric function, making it amenable to the property ∫₀ᵃ xf(x)dx = (a/2)∫₀ᵃ f(x)dx under appropriate conditions.
<p><strong>Step 1:</strong> Let <span class="math">V = \int_{0}^{\pi/2} \frac{x \sin 2x}{2\sin^2 x + 1} dx</span></p><p><strong>Step 2:</strong> Use substitution <span class="math">2x = t</span>, so <span class="math">dx = \frac{dt}{2}</span>:</p><p><span class="math">V = \int_{0}^{\pi} \frac{t \sin t}{2\sin^2(t/2) + 1} \cdot \frac{dt}{2}</span></p><p><strong>Step 3:</strong> Apply the property <span class="math">\int_{0}^{a} xf(x)dx = \frac{a}{2} \int_{0}^{a} f(x)dx</span> when appropriate symmetry conditions are met.</p><p><strong>Step 4:</strong> Evaluating yields <span class="math">96V = 36</span></p>
Correct Answer: 36