Differential Equations
Variable Separable Method
Grade 12

Question:

<p>If \(\dfrac{dy}{dx} = y + 3 > 0\) and \(y(0) = 2\), then \(y(\ln 2)\) is equal to</p>
<p>5</p>
<p>13</p>
<p>\(-2\)</p>
<p>7</p>

Step-by-Step Solution

Key Concept: This is a separable first-order linear ODE. Separate variables as dy/(y+3) = dx, integrate both sides, and use the initial condition to find the particular solution.
<p><strong>Step 1:</strong> Separate variables</p><p>Given: dy/dx = y + 3 with y(0) = 2</p><p>Rearranging: dy/(y + 3) = dx</p><p><strong>Step 2:</strong> Integrate both sides</p><p>∫ dy/(y + 3) = ∫ dx</p><p>ln|y + 3| = x + C</p><p><strong>Step 3:</strong> Apply initial condition y(0) = 2</p><p>ln|2 + 3| = 0 + C</p><p>ln(5) = C</p><p><strong>Step 4:</strong> Write the particular solution</p><p>ln|y + 3| = x + ln(5)</p><p>|y + 3| = e^(x + ln 5) = 5e^x</p><p>Since y(0) = 2 > -3, we have y + 3 > 0, so:</p><p>y + 3 = 5e^x</p><p>y = 5e^x - 3</p><p><strong>Step 5:</strong> Evaluate at x = ln(2)</p><p>y(ln 2) = 5e^(ln 2) - 3 = 5(2) - 3 = 10 - 3 = 7</p><p>∴ Answer: <strong>7</strong></p>
Correct Answer: D

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