Differential equation for $y=A\cos αx+B\sin αx$, where A and B are the arbitrary constants, is:

$\frac{d^2y}{dx^2}-α^2y=0$ $\frac{d^2y}{dx^2}+α^2y=0$ $\frac{d^2y}{dx^2}+αy=0$ $\frac{d^2y}{dx^2}-αy=0$

$\frac{d^2y}{dx^2}+α^2y=0$

$\frac{d^2y}{dx^2}=-α^2y⇒\frac{d^2y}{dx^2}+α^2y=0$