\[\begin{array}{1 1}(1)\frac{d^{2}y}{dx^{2}}+ay=0&(2)\frac{d^{2}y}{dx^{2}}-9y=0\\(3)\frac{d^{2}y}{dx^{2}}-9\frac{dy}{dx}=0&(4)\frac{d^{2}y}{dx^{2}}+9x=0\end{array}\]