\( G(x) \frac{d^2 u}{dx^2} + P(x) \frac{du}{dx} + Q(x) u = 0 \qquad (1) \) \begin{equation} \textnormal{If } u_1(x) \textnormal{ and } u_2(x) \textnormal{ are both solutions of the linear homogeneous equation (1) and } h_1 \textnormal{ and } h_2 \textnormal{ are constants, then the function} u(x) = h_1 u_1(x) + h_2 u_2(x) \textnormal{ is also a solution of equation (1).} \end{equation}