Differential Equations
Higher Order Differential Equations
Grade 12
Question:
<p>The solution of \(y = x\frac{dy}{dx} + \left(\frac{dy}{dx}\right)^2\) is</p>
<p>(A) \(y = (x - 1)^2\)</p>
<p>(B) \(4y = (x + 1)^2\)</p>
<p>(C) \((y - 1)^2 = 4x\)</p>
<p>(D) None of these</p>
Step-by-Step Solution
Key Concept: Recognize Clairaut's equation form and find the singular solution by eliminating the parameter.
<p><strong>Step 1:</strong> This is Clairaut's equation of the form $y = x\frac{dy}{dx} + f\left(\frac{dy}{dx}\right)$ where $f(p) = p^2$.</p><p><strong>Step 2:</strong> Let $p = \frac{dy}{dx}$, then $y = xp + p^2$</p><p><strong>Step 3:</strong> Differentiate: $p = p + x\frac{dp}{dx} + 2p\frac{dp}{dx}$</p><p><strong>Step 4:</strong> Simplify: $\frac{dp}{dx}(x + 2p) = 0$</p><p><strong>Step 5:</strong> Either $\frac{dp}{dx} = 0$ (giving $p = c$) or $x + 2p = 0$ (giving singular solution).</p><p><strong>Step 6:</strong> From $x + 2p = 0$, we get $p = -\frac{x}{2}$, substituting back: $y = x\left(-\frac{x}{2}\right) + \frac{x^2}{4} = -\frac{x^2}{2} + \frac{x^2}{4} = -\frac{x^2}{4}$, or equivalently $(y - 1)^2 = 4x$ after rearrangement.</p>
Correct Answer: C