# Homework Help: Second order linear system and power series: Differential Equations

1. Dec 8, 2009

### clarineterr

1. The problem statement, all variables and given/known data
Find a third degree polynomial approximation for the general solution to the differential equation:

$$\frac{d^{2}y}{dt^{2}}$$ +3$$\frac{dy}{dt}$$+2y= ln(t+1)

2. Relevant equations
Power series expansion for ln(t+1)

3. The attempt at a solution

The system to the corresponding homogeneous equation $$\frac{d^{2}y}{dt^{2}}$$ +3$$\frac{dy}{dt}$$+2y = 0

is y(t) = k1e-t +k2e-2t

Then I guessed$$\frac{ at^{3}}{3}$$-$$\frac{bt^{2}}{2}$$+ct as a solution for the original equation. Plugging this in I got a=1/2, b=2,c=2/3

But then I still have the t$$^{4}$$, t$$^{5}$$ terms, etc left in the equation. Im not quite sure how a third degree polynomial can be a solution to this equation.

2. Jan 3, 2010

### NSAC

Because it will be an approximation not really the solution itself.