Solve the IVP $xy'' + y' + 4xy = 0, y(0) = 3, y'(0) = 0$

805 Views Asked by At

It has to be solved with Laplace transform and then converted to Bessel equation.

$L(xy'') = -\frac{dL(y'')}{ds}$

$L(4xy) = -\frac{4dL(y)}{ds}$

$L(y'') = s²L(y) - sy(0) - y'(0) = s²L(y) -3s$

$L(y') = sL(y) - sy(0) - y(0) = sL(y) - 3$

$-\frac{d(s²L(y)-3s)}{ds} + sL(y)-\frac{4dL(y)}{ds} =0$

$-\frac{d(s²L(y))}{ds} + 3 + sL(y) - 3 -\frac{4dL(y)}{ds} =0$

$-\frac{s²L(y)}{ds} + sL(y) -\frac{4dL(y)}{ds} =0$

$-\frac{L(y)(s²+4)}{ds} + sL(y) =0$ (1)

$\frac{dL(y)}{L(y)} = \frac{sds}{s²+4}$

Integrating both sides

$ln(L(y)) = \frac{ln(s²+4)}{2} + c$

$L(y) = c\sqrt{s²+4}$

which won't lead me to the right answer.

I realized that if at (1) I use $\frac{L(y)(s² + 4)}{ds} + sL(y) =0$ instead I'll get the right answer according to wolfram, but I can't see what I'm doing wrong to end up with that negative sign.

http://www.wolframalpha.com/input/?i=xy%27%27+%2B+y%27+%2B+4xy+%3D+0%2C+y%280%29+%3D+3%2C+y%27%280%29+%3D+0

1

There are 1 best solutions below

0
On

Managed to find my mistake, I solved as if s² wasn't part of the derivative at $-\frac{d(s²L(y))}{ds}$

$-\frac{d(s²L(y))}{ds} = -2sL(y) - \frac{s²d(L(y))}{ds}$ $-2sL(y) - \frac{s²d(L(y))}{ds} + sL(y) -\frac{4dL(y)}{ds} =0$ $ - \frac{s²d(L(y))}{ds} - sL(y) -\frac{4dL(y)}{ds} =0$ $ \frac{s²d(L(y))}{ds} + sL(y) +\frac{4dL(y)}{ds} =0$ $ \frac{d(L(y))(s²+4)}{ds} + sL(y) =0$