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streetyfatb
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#1
Report Thread starter 14 years ago
#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
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BCHL85
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#2
Report 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
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