Solve d²y/dx² + y = 0 (SHM):
Ay = e^x
By = A cos x + B sin x (or A sin(x + φ))
Cy = x²
Dy = ln x
Answer & Solution
Correct answer: B. y = A cos x + B sin x (or A sin(x + φ))
Auxiliary equation: m² + 1 = 0 → m = ±i. General solution: y = A cos x + B sin x. Two-parameter family (2nd-order needs 2 conditions). Equation of simple harmonic motion.
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