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yaro99
Steady state solution 4y''+12y'+9y=225-75sin(3t) I'm not sure what to do about the 225, I'd be able to solve it if it was just the sine term on the right.
Just add another constant in your guess for the particular if you are using undefined variables.
So it would be in the form \[K+a*\cos(3t)+b*\sin(3t)\]?
Cool, I got the solution. Thanks!