# 2nd order Linear DE

1. Jan 17, 2013

### Apteronotus

Hi,

When solving a 2nd order Linear DE with constant coefficients ($ay''+by'+cy=0$) we are told to look for solutions of the form $y=e^{rt}$ and then the solution (if we have 2 distinct roots of the characteristic) is given by
$y(t)=c_1 e^{r_1 t}+c_2 e^{r_2 t}$

This is clearly a solution, but how do we know there are no other solutions?
That is, how do we know this is the general solution?

2. Jan 17, 2013

### tiny-tim

Hi Apteronotus!
It's easy to prove for the first-order case …

if y' - ry = 0, put y = zert, then (z' + rz)ert = rzert

so ert = 0 (which is impossible),

or z' + rz = rz, ie z' = 0, ie z is constant

and now try (y' - ry)(y' - sy) = 0, using the same trick twice