Example 1.11
Solve the following second order ODE:
u − 5u + 6u = 6x + 7. (1.63)
We first find uc. The auxiliary equation for the related homogeneous equation
is given by
r2 − 5r + 6 = 0, (1.64)
with roots given by
r = 2,3. (1.65)
The general solution is given by
u(x) = αe2x + βe3x . (1.66)
Noting that g(x) = 6x + 7, then a particular solution is assumed to be of the
form
up = Ax + B. (1.67)
Since this is a particular solution, then substituting up into the inhomoge-
neous equation leads to
6Ax + (6B − 5A) = 6x + 7. (1.68)
Equating the coefficients of like terms from both sides gives
A = 1, B = 2. (1.69)
This in turn gives
u(x) = uc + up = αe2x + βe3x + x + 2, (1.70)
where α and β are arbitrary constants.