Example 1.9
Solve the following second order ODE:
u − u = 0. (1.51)
The auxiliary equation is given by
r2 − 1 = 0, (1.52)
and this gives
r = ±1. (1.53)
Accordingly, the general solution is given by
u(x) = Aex + Be−x . (1.54)
It is interesting to point out that the normal form ODE:
u + u = 0, (1.55)
leads to the auxiliary equation:
r2 + 1 = 0, (1.56)
and this gives
r = ±i. (1.57)
The general solution is given by
u(x) = A cos x + B sin x. (1.58)