# How do I "guess" the solution to y to this D.E.

• Oct 30th 2012, 01:36 PM
fb280
How do I "guess" the solution to y to this D.E.
y"-4y'+3y = 8xex

I guessed that the solution to y was Axex but when I plugged it into y it did not get me the correct answer. Please don't work out the problem for me, right now I just need help understanding how I would guess y in this situation. Much appreciated.
• Oct 30th 2012, 02:37 PM
TheEmptySet
Re: How do I "guess" the solution to y to this D.E.
Quote:

Originally Posted by fb280
y"-4y'+3y = 8xex

I guessed that the solution to y was Axex but when I plugged it into y it did not get me the correct answer. Please don't work out the problem for me, right now I just need help understanding how I would guess y in this situation. Much appreciated.

Since the complimentary solution is $y_c=c_1e^{x}+c_2e^{3x}$

You would normally gues the solution is

$y=Ae^{x}+Bxe^{x}$

But since the first term is part of the complimentary solution we need to multiply the particular soluiton by $x$ to get the guess

$y_p=Axe^{x}+Bx^2e^{x}$
• Oct 30th 2012, 06:09 PM
fb280
Re: How do I "guess" the solution to y to this D.E.
why would the guess normally be Aex for one part instead of Axex? And why is there a B? If there was a B I would have thought that it would be Bex and not Bxex
• Oct 30th 2012, 07:41 PM
MarkFL
Re: How do I "guess" the solution to y to this D.E.
On the right side of the ODE you have the product of a linear function and an exponential function, so you would expect the particular solution would take the form:

$y_p=(Ax+B)e^x$

Note: while I have switched A and B from the form The EmptySet used, they are equivalent.

As stated, you don't want any of the terms of your particular solution to be part of the complementary solution, so you multiply the particular solution by $x^s$ where $s$ is the smallest integer so that no terms of the particular solution are part of the complementary solution. In this case, $s=1$. Hence:

$y_p=x(Ax+B)e^x=Ax^2e^x+Bxe^x$
• Oct 30th 2012, 08:33 PM
fb280
Re: How do I "guess" the solution to y to this D.E.
Okay I think I understand what I was confused on before. But when I plug in Ax2ex+Bxex I got stuck at the part where it turns out to be

-2A2xex+A2ex+Bx4ex-4Bx2ex+3Bxex=8xex

Ax2ex-4Ax2ex+3Ax2ex cancels out so I did not include this part above
• Oct 30th 2012, 09:47 PM
MarkFL
Re: How do I "guess" the solution to y to this D.E.
Using:

$y_p=Ax^2e^x+Bxe^x$

we find:

$y_p'=e^x(Ax^2+2Ax+Bx+B)$

$y_p''=e^x(Ax^2+4Ax+Bx+2A+2B)$

and substituting into the ODE, we get:

$e^x(Ax^2+4Ax+Bx+2A+2B)-4e^x(Ax^2+2Ax+Bx+B)+3e^x(Ax^2+Bx)=8xe^x$

Divide though by $e^x$, distribute constants and collect like terms:

$Ax^2+4Ax+Bx+2A+2B-4(Ax^2+2Ax+Bx+B)+3(Ax^2+Bx)=8x$

$Ax^2+4Ax+Bx+2A+2B-4Ax^2-8Ax-4Bx-4B+3Ax^2+3Bx=8x$

$(-4A)x+(2A-2B)=8x+0$

Equating coefficients yields the system:

$-4A=8$

$2A-2B=0$

Now, solve this system to determine A and B, and then you have your particular solution.
• Oct 31st 2012, 05:07 AM
fb280
Re: How do I "guess" the solution to y to this D.E.
okay I understand where I went wrong now. thanks for replying, this really helped me a lot.