In a differential equations class the professor stated that the general solution of a homogeneous second-order linear ODE would be in the form:
$$y = c_1y_1 + c_2y_2$$
Where $y_1$ and $y_2$ were distinct solutions of the ODE:
$$\frac{d^2y}{dx^2} + A\frac{dy}{dx} + By = 0$$
Where $A$ and $B$ are constant coefficients. I understand how to get the distinct solutions however what i don't get is why the general solution is in the form it is and what is the proof behind it?
Best Answer
Take a solution $y$, and for now assume that $y=c_1y_1+c_2y_2$. Consider $y(t_0)=y_0$ and $y'(t_0)=y_0'$. Thus we have $$y(t_0)=c_1y_1(t_0)+c_2y_2(t_0)=y_0$$ $$\text{and}$$ $$y'(t_0)=c_1y_1'(t_0)+c_2y_2'(t_o)=y_0'$$ We now must solve for $c_1$ and $c_2$. We have the linear system $$\left(\begin{array}{cc} y_1(t_0) & y_2(t_0) \\ y_1'(t_0) & y_2'(t_0) \end{array}\right) \left(\begin{array}{c} c_1 \\ c_2 \end{array}\right)= \left(\begin{array}{c} y_0 \\ y_0' \end{array}\right)\qquad (1)$$ Now since $y_1$ and $y_2$ are fundamental solutions, meaning their Wronskian $$W[y_1,y_2]=\left| \begin{array}{cc} y_1(t) & y_2(t) \\ y_1'(t) & y_2'(t) \end{array} \right|\neq 0$$ $\forall t$ in the interval upon which $y_1$ and $y_2$ are being taken as solutions we have the explicit solution to $(1)$ which is $$\left(\begin{array}{c} c_1 \\ c_2 \end{array}\right)=\frac{1}{W[y_1,y_2](t_0)}\left(\begin{array}{cc} y_2'(t_0) & -y_2(t_0) \\ -y_1'(t_0) & y_1(t_0) \end{array}\right)\left(\begin{array}{c} y_0 \\ y_0' \end{array}\right)$$ Hence there exists a $c_1$ and $c_2$ such that $y=c_1y_1+c_2y_2$ and by the Existence-Uniqueness Theorem this is the only possible $y$ such that $y(t_0)=y_0$ and $y'(t_0)=y_0'$, forcing $y$ to be of this form. QED
Above I gave the proof for why all solutions to the homogeneous linear ODE must to be of the form $y=c_1y_1+c_2y_2$. But if you only wish to convince yourself why $y=c_1y_1+c_2y_2$ is a solution then realize that since the operator $L[y]$, $$L[y]=y''+Ay'+By$$ is linear we have $$L[c_1y_1+c_2y_2]=L[c_1y_1]+L[c_2y_2]=c_1L[y_1]+c_2L[y_2]=0$$ because, since $y_1$ and $y_2$ are solutions to the DE, $L[y_1]=L[y_2]=0$ and since $L[c_1y_1+c_2y_2]=0$ it is also a solution to the DE.