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Multiple Choice
Solve the following initial-value problem using Laplace transforms: , , . What is ?
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Verified step by step guidance
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Step 1: Apply the Laplace transform to the differential equation y'' + 4y = 0. Recall that the Laplace transform of y'' is \( s^2Y(s) - sy(0) - y'(0) \), and the Laplace transform of y is \( Y(s) \). Substitute the initial conditions \( y(0) = 2 \) and \( y'(0) = 0 \).
Step 2: After applying the Laplace transform, the equation becomes \( s^2Y(s) - 2 + 4Y(s) = 0 \). Rearrange the equation to isolate \( Y(s) \): \( Y(s)(s^2 + 4) = 2 \). Solve for \( Y(s) \): \( Y(s) = \frac{2}{s^2 + 4} \).
Step 3: Recognize that \( \frac{2}{s^2 + 4} \) is the Laplace transform of \( 2\cos(2t) \). This is based on the standard Laplace transform pair \( \mathcal{L}(\cos(at)) = \frac{s}{s^2 + a^2} \) and \( \mathcal{L}(\sin(at)) = \frac{a}{s^2 + a^2} \).
Step 4: Take the inverse Laplace transform of \( Y(s) \) to find \( y(t) \). Using the result from Step 3, \( y(t) = 2\cos(2t) \).
Step 5: Verify the solution by substituting \( y(t) = 2\cos(2t) \) back into the original differential equation \( y'' + 4y = 0 \). Compute \( y'' \) and check that the equation holds true, confirming the correctness of the solution.