# diffEQ using L, D, Ly, y(x)... not sure how to approach this

• Feb 24th 2010, 06:18 PM
plopony
diffEQ using L, D, Ly, y(x)... not sure how to approach this
If L = D^2 + 3xD - 4x

and y(x) = 2x - 5e^(2x)

then Ly = ?

This is completely going over my head. I don't know what is being asked. Im not asking someone to solve this for me, just some instruction in how I am supposed to approach this. Thanks
• Feb 24th 2010, 07:06 PM
TheEmptySet
Quote:

Originally Posted by plopony
If L = D^2 + 3xD - 4x

and y(x) = 2x - 5e^(2x)

then Ly = ?

This is completely going over my head. I don't know what is being asked. Im not asking someone to solve this for me, just some instruction in how I am supposed to approach this. Thanks

For example if $y=x+\sin(x)$ then

$Ly=(D^2+3xD-4x)(x+\sin(x))$

Now distribute the operator to y to get

$(D^2+3xD-4x)x+(D^2+3xD-4x)\sin(x)$

distributing again gives

$D^2(x)+3xD(x)-4x(x)+D^2(\sin(x))+3xD(\sin(x))-4x(\sin(x))$

Now just take the dervaitves to get

$0+3x(1)-4x^2-\sin(x)+3x(\cos(x))-4x\sin(x)$

I hope this helps