Second order differential equation

  • Thread starter doroulla
  • Start date
  • #1
16
0
hi. I cant figure out this question:

d2y/dx2 - 2 dy/dx - 3y = x

(i) find complementary function
(ii) find particular integral
(iii) using (i) and (ii) find the general solution
(iv) find the solution that satisfies the initial conditions:
y=2/9 at x=0 and dy/dx=-13/3 at x=0


i did:

m^2 - 2m - 3 = 0
(m-3)(m+1)=0
real and distinct solutions thus

y = Ae^(3x) + Be^(-x)

thus dy/dx = 3Ae^3x - Be^-x

d2y/dx2 = 9Ae^3x + Be^-x

now i have no idea how to continue. As i understood what i found above is the complementary function. I think. Thank you
 

Answers and Replies

  • #2
rock.freak667
Homework Helper
6,223
31
y = Ae^(3x) + Be^(-x)

Yes this would be the complementary solution.

What the particular integral for a polynomial?
 
  • #3
16
0
i think its something to do with the x on the right hand side but i dont know how to do it. Do i integrate that side?

I know when you have a y in the equation you integrate the constants infront of the y. But in this case i dont have this.
 
  • #4
HallsofIvy
Science Advisor
Homework Helper
41,847
966
Try a specific integral of the form [itex]Ax^2+ Bx+ C[/itex]. What must A, B, and C equal to satisfy the equation? Recall that the general solution to the entire equation is the general solution to the associated homogeneous equation (the "complementary solution") plus a specific integral.
 

Related Threads on Second order differential equation

  • Last Post
Replies
1
Views
504
  • Last Post
Replies
5
Views
3K
  • Last Post
Replies
3
Views
1K
  • Last Post
Replies
1
Views
979
  • Last Post
Replies
2
Views
1K
  • Last Post
Replies
2
Views
278
  • Last Post
Replies
1
Views
1K
  • Last Post
Replies
5
Views
938
  • Last Post
Replies
1
Views
980
  • Last Post
Replies
3
Views
1K
Top