Undetermined Coefficients

  • #1

Homework Statement


Set up but do not solve for the appropriate particular solution yp for the differential equation
y''+25y=2xsin(5x)
using the Method of Undetermined Coefficients (primes indicate derivatives with respect to x).

Homework Equations





The Attempt at a Solution


I first solved the associated homogeneous equation and got the equation r^2+25r=0 and the roots are 0 and -5. So the equation becomes C1+C2e^(-5x).

I separated 2x as (Ax+B) then seeing as it's being multiplied by sin (5x), I can just do (Ax+B)cos(5x)+(Cx+D)sin(5x). I check this solution with my homogeneous and there are no repeated terms. I tried entering this solution in (Ax+B)cos(5x)+(Cx+D)sin(5x and it keeps telling me I'm wrong. Am I doing something wrong or forgetting a step or two?

Am I doing something horrificly wrong?
 
Last edited:

Answers and Replies

  • #2
vela
Staff Emeritus
Science Advisor
Homework Helper
Education Advisor
15,136
1,732
One of your roots is wrong. It should be -25, not -5. Other than that, it looks good so far.
 
  • #3
Yeah sorry, I was missing the first half of the question, if it says setup but don't solve for the coefficients for just the particular solution, is what I have correct because it keeps saying i'm wrong and I have no idea. Am I missing some steps?
 
  • #4
vela
Staff Emeritus
Science Advisor
Homework Helper
Education Advisor
15,136
1,732
It looks okay to me. Are you sure the LHS of the differential equation is y''+25y' and not y''+25y? That would change your particular solution a bit.
 
  • #5
oh ****.. yeah it's 25y.. but how does it change my particular solution? o_O
 
Last edited:

Related Threads on Undetermined Coefficients

  • Last Post
Replies
1
Views
1K
  • Last Post
Replies
5
Views
1K
  • Last Post
Replies
3
Views
1K
  • Last Post
Replies
1
Views
1K
  • Last Post
Replies
2
Views
1K
  • Last Post
Replies
5
Views
1K
  • Last Post
Replies
3
Views
2K
  • Last Post
Replies
4
Views
886
  • Last Post
Replies
3
Views
4K
  • Last Post
Replies
2
Views
2K
Top