# P5 - 2nd Diff Equ...Watch

Announcements
This discussion is closed.
#1
Hi,

just going through this qu:

d2y/dx^2 - 3dy/dx - 4y = 50sin2x

I get y = Ae^4x + Be^-x + 3cos2x - 4sin2x

I am told x=0 y=0 thus A + B = -3

and as x tends to infinite y is finate, eventually and i need to find y in terms of x, this part is where im stuck, i am aware that A and B will have to be solved simultaniously, but what happens when x tends to infinite and y is finate?

Many Thanks

Streety
0
14 years ago
#2
Suppose A <> 0.
If x -> infinite, e^-x tends to 0, e^4x tends to infinite.
So Ae^4x + Be^-x tends to infinite -> y tends to infinite.
Therefore A = 0, so B = -3
0
X
new posts
Back
to top
Latest
My Feed

### Oops, nobody has postedin the last few hours.

Why not re-start the conversation?

see more

### See more of what you like onThe Student Room

You can personalise what you see on TSR. Tell us a little about yourself to get started.

### University open days

• University of Surrey
Wed, 23 Oct '19
• University of Bristol
Wed, 23 Oct '19
• University of Exeter
Wed, 23 Oct '19

### Poll

Join the discussion

Yes (31)
25%
No (93)
75%