Differential Equations
First Order Linear Differential Equations
Grade 12
Question:
<p>The family of curves represented by <span>\(\frac{dy}{dx} = \frac{x + y}{x}\)</span> is</p>
<p>(A) <span>\(y = x \ln(x) + Cx\)</span></p>
<p>(B) <span>\(y = x \ln(x) - Cx\)</span></p>
<p>(C) <span>\(2y = e^{x^2}\)</span></p>
<p>(D) None of these</p>
Step-by-Step Solution
Key Concept: Recognize the linear differential equation structure and apply the integrating factor method to solve for the family of curves.
<p><strong>Step 1:</strong> Rewrite the differential equation: <span>$\frac{dy}{dx} = 1 + \frac{y}{x}$</span></p><p><strong>Step 2:</strong> This is a linear differential equation of the form <span>$\frac{dy}{dx} - \frac{y}{x} = 1$</span></p><p><strong>Step 3:</strong> The integrating factor is <span>$e^{\int -\frac{1}{x}dx} = e^{-\ln x} = \frac{1}{x}$</span></p><p><strong>Step 4:</strong> Multiply both sides by the integrating factor: <span>$\frac{1}{x}\frac{dy}{dx} - \frac{y}{x^2} = \frac{1}{x}$</span></p><p><strong>Step 5:</strong> The left side is <span>$\frac{d}{dx}\left(\frac{y}{x}\right) = \frac{1}{x}$</span></p><p><strong>Step 6:</strong> Integrate both sides: <span>$\frac{y}{x} = \ln(x) + C$</span></p><p><strong>Step 7:</strong> Therefore <span>$y = x\ln(x) + Cx$</span></p><p>∴ Answer is A.</p>
Correct Answer: A