Higher order of derivatives: variation of parameters?
$$y'''+y' = \tan t, \quad 0 < t< \pi.$$
Use variation of parameters to determine the general solution of the given differential equation.
I know the roots of the characteristic equation are $0$, $-i$, and $i$. But I'm really confused about how to approach the question. I tried doing the Wronskian but because it has a double root, it gave me a weird answer. Can someone please help me?
By the way, the answer of the question is $$y= c_1 + c_2 \cos t + c_3 \sin t - \ln \cos t - \sin t \ln(\sec t + \tan t).$$