I'm using the method of undetermined coefficients to find a particular solution of: y''+y'=xe^{-x

Agohofidov6

Agohofidov6

Answered question

2022-01-18

I'm using the method of undetermined coefficients to find a particular solution of: y+y=xex
Ostensibly, it seems that yp should take the form of (Ax+B)ex
At least that's the form that I think I've been taught. Problem is that it just doesn't work out for me. I get a value for A, but not for B... Am I choosing an incorrect yp form?

Answer & Explanation

Cassandra Ramirez

Cassandra Ramirez

Beginner2022-01-19Added 30 answers

Its
alkaholikd9

alkaholikd9

Beginner2022-01-20Added 37 answers

As already suggested, you may look for a solution of the form
yp(x)=(Ax2+Bx)ex
(You may also consider yp(x)=(Ax2+Bx+C)ex, to find out that C cancels anyway.) Then,
yp(x)=(2Ax+B)ex(Ax2+Bx)ex
and yp(x)=2Aex(2Ax+B)ex(2Ax+B)ex+(Ax2+Bx)ex,
from which you get
yp(x)+yp(x)=ex(2A2AxB2AxB+Ax2+Bx+2Ax+BAx2Bx)
=ex(2A2AxB)
Comparing 2A2AxB to x, we get 2AB=0 and 2A=1, hence A=12 and B=1. Thus
yp(x)=(x22x)ex
alenahelenash

alenahelenash

Expert2022-01-24Added 556 answers

The characteristic polynomial here is r2+r=r(r+1). The root r=1 appears here with multiplicity m=1, so you multiply your suggested solution (Ax+B)ex by xm=x and try yp=(Ax2+Bx)ex instead.

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