# Finding steady-state solution to 2nd-order DE

• shreddinglicks
In summary, the conversation discusses finding the particular solution for a second order equation and its derivatives, and ultimately solving for the unknowns A and B in the equation. The process involves identifying the factors of sin(xt) and cos(xt) on both sides of the equation and solving for A and B.

## Homework Statement

I want to get the particular solution for:

Lq''+Rq'+(1/c)q = Esin(xt)

## Homework Equations

Lq''+Rq'+(1/c)q = Esin(xt)

## The Attempt at a Solution

I got the particular solution and its derivatives:

q = A*Sin(xt) + B*cos(xt)
q' = x*A*cos(xt) - x*B*sin(xt)
q'' = -x^2*A*sin(xt) - x^2*B*cos(xt)

I plug these into the 2nd order equation giving me:

A[-x^2*L*sin(xt) + R*x*cos(xt) + (1/c)*Sin(xt)] + B[-x^2*L*cos(xt)-x*R*sin(xt)+(1/c)*cos(xt)] = Esin(xt)

I want to get the form of A and B in the attached image

Also, alpha = x in my typing.

#### Attachments

• Captdure.PNG
36.4 KB · Views: 593
Identifythe factors of sin(xt) on both sides of the wquation and then do the same for cos(xt). That gives you two equations for two unknowns.

Orodruin said:
Identifythe factors of sin(xt) on both sides of the wquation and then do the same for cos(xt). That gives you two equations for two unknowns.

I'm not sure if I understand. So I factor out sin(xt) on both sides and factoring cos(xt) giving me:

sin(xt)*[-L*x^2*A - R*x*B + (1/c)*A] + cos(xt)*[-L*x^2*B + R*x*A + (1/c)*B]

That's just one side of the equation. You need to do it on both sides and then identify the factors on either side.

Orodruin said:
That's just one side of the equation. You need to do it on both sides and then identify the factors on either side.

But the other side is just Esin(xt).

So I have:

(-L*x^2*B) + (R*x*A)+([1/c]*B) = 0
(-L*x^2*A) - (R*x*B) + ([1/c]*A) = E

Right. Solve for A and B.

Orodruin said:
Right. Solve for A and B.

I got the answer, thank you.