Differential Equations
Linear Differential Equations
Grade 12
Question:
<p>Find the solution of the differential equation <span class="math">x\frac{dy}{dx} + y - x + xy\cot x = 0</span>, where <span class="math">x \neq 0</span></p>
<p>(a) <span class="math">y = x - \cot x + \frac{C}{\sin x}</span></p>
<p>(b) <span class="math">y = \frac{1}{x} - \cot x - \frac{C}{x\sin x}</span></p>
<p>(c) <span class="math">y = \frac{1}{x} - \cot x + \frac{C}{x\sin x}</span></p>
<p>(d) <span class="math">y = \frac{1}{x} + \cot x - \frac{C}{\sin x}</span></p>
Step-by-Step Solution
Key Concept: Convert the equation into standard linear form by dividing by x, then apply the integrating factor method.
<p><strong>Step 1:</strong> Given equation: <span class="math">x\frac{dy}{dx} + y - x + xy\cot x = 0</span></p><p><strong>Step 2:</strong> Rearrange to get:</p><p><span class="math">x\frac{dy}{dx} + y(1 + x\cot x) = x</span></p><p><strong>Step 3:</strong> Divide both sides by <span class="math">x</span>:</p><p><span class="math">\frac{dy}{dx} + \frac{y(1 + x\cot x)}{x} = 1</span></p><p><strong>Step 4:</strong> This is a linear differential equation of the form <span class="math">\frac{dy}{dx} + P(x)y = Q(x)</span>, where <span class="math">P(x) = \frac{1 + x\cot x}{x}</span> and <span class="math">Q(x) = 1</span></p><p><strong>Step 5:</strong> Find the integrating factor: <span class="math">\mu(x) = e^{\int P(x)dx}</span></p><p><strong>Step 6:</strong> The solution is <span class="math">y = \frac{1}{x} - \cot x + \frac{C}{x\sin x}</span></p><p>∴ Answer is C.</p>
Correct Answer: C