- #1

Lancelot59

- 646

- 1

[tex]y''-y'+\frac{y}{4}=3+e^{\frac{x}{2}}[/tex]

and asked to solve it using undetermined coefficients. Using the auxilary equation

[tex]y=e^{\lambda t}[/tex]

I got [tex]y_{1}=e^{\frac{x}{2}}, y_{1}=xe^{\frac{x}{2}}[/tex]

Now to solve the particular solution, I chose to guess:

[tex]y_{p}=axe^{\frac{x}{2}}+b[/tex] and [tex]y_{p}=cx+d[/tex]

From what I understand in this case, you're supposed to guess a polynomial of one order higher than what you have. However I basically get a giant mess:

[tex]y=axe^{\frac{x}{2}}+b+cx+d[/tex]

[tex]y'=ae^{\frac{x}{2}}+a\frac{x}{2}e^{\frac{x}{2}}+c[/tex]

[tex]y''=a\frac{x}{4}e^{\frac{x}{2}}+ae^{\frac{x}{2}}[/tex]

Gives:

[tex]a\frac{x}{4}e^{\frac{x}{2}}+ae^{\frac{x}{2}}-ae^{\frac{x}{2}}+a\frac{x}{2}e^{\frac{x}{2}}+c+ \frac{1}{4}(axe^{\frac{x}{2}}+b+cx+d)=3+e^{x}{2}[/tex]

I can't compare coefficients for all of the terms. Some of the exponentials are multiplied by x, which doesn't appear on the other side. What am I supposed to do?