# Homework Help: Differential Equations - Method of Undetermined Coefficients

1. Sep 25, 2011

### MJay82

1. The problem statement, all variables and given/known data
I've been having problems with a number of these things, here's the first one:
y'' -2y' -3y = -3te-t

2. Relevant equations
I know that the general solution will be
y = yh + yp
where yh is the general solution to the homogeneous equation, and yp
is the particular solution of the non-homogeneous equation.

3. The attempt at a solution
I got the homogeneous solutions very easily, but I'm tricked by how to solve for yp. I understand the principle of the solution if the right side of the equation was simply -3e-t, but when the extra t is thrown in there, my understanding breaks down.

I tried yp=t2e-t, differentiated twice, and then input these expressions into the equation. I just realized that I was supposed to use an unknown coefficient (A we'll call it) with my guess at yp. Since the terms on the right side are a product, will I just use A, or will I need some B as well? Thanks for any help.

2. Sep 25, 2011

### ehild

You need to use unknown coefficients, in plural . Try the solution in the form yp=(at2+bt+c)e-t.

ehild

3. Sep 25, 2011

### HallsofIvy

IF $e^{-t}$ were not already a solution to the associated homogeneous equation, since the right side is $te^{-t}$, you would try $y_p= (At+ B)e^{-t}$. Because $e^{-t}$ IS a solution to the associated homogeneous equation, you should try $y_p= (At^2+ Bt)e^{-t}$.

(You don't really need the "c" in ehild's suggestion. It wouldn't hurt, but you would find that c= 0.)

4. Sep 25, 2011

### MJay82

Thanks y'all! I'll be trying a little later this afternoon.