Differential Equations

The general solution of the second-order ODE $\frac{d^2y}{dx^2} + 9y = 0$ is:

Options:

• (A) $y = C_1 \cos(3x) + C_2 \sin(3x)$
• (B) $y = C_1 e^{3x} + C_2 e^{-3x}$
• (C) $y = (C_1 + C_2 x) e^{3x}$
• (D) $y = C_1 \cos(9x) + C_2 \sin(9x)$