# Non-homogenous differential Equation

1. Feb 27, 2013

### sndoyle1

1. The problem statement, all variables and given/known data
solve:
y""+6y'+9y=e-3x/x3

2. Relevant equations

y=yc+yp

3. The attempt at a solution

I found yc=C1e-3x+C2xe-3x
and am having difficulties finding yp. I am wondering which method would be the best to determine yp:

- annihilators
- undetermined coefficients
- variation of paramaters.

2. Feb 27, 2013

### sandy.bridge

Since it is in the form $e^{ax}/x^k$ try using $Ae^{-3x}/x$

3. Feb 27, 2013

### sndoyle1

Thanks, it worked out. I have a hard time knowing what 'guess' to use for the derivative. How did you know to put it over x instead of x-3? I have a test tomorrow, so I want to make sure that I can do things properly.

4. Feb 27, 2013

### sandy.bridge

I usually always try the simplest first. This doesn't pertain to this question, but if $Ae^{ax}$ didn't work I would try $Axe^{ax}$, and if that didn't work I would try $Ax^2e^{ax}$. It can be rather tedious for some questions but eventually you start to notice patterns.

5. Feb 28, 2013

### HallsofIvy

Is that really a fourth degree equation or is the second '' a typo?

"Undetermined coefficents" works when the right side of the equation is one of the types of solutions you can get as solutions to homogenous differential equations with constant coefficients: exponentials, sine or cosine, and polynomials, as well as combinations of those. That is not the case here. I recommend "variation of parameters".

6. Feb 28, 2013

### sandy.bridge

I think he accidentally hit the quotation mark key.