# How to solve: $y'+\frac yt=3\cos2t$, $t>0$ (using variation of parameters)?

How to solve: $y'+\frac yt=3\cos2t$, $t>0$ (using variation of parameters)? I know the solution using the integrating factor method, which is $\frac ct+\frac{3\cos2t}{4t}+\frac{3\sin2t}2$.

The given differential equation can be written as $$(t\>y)'=3t\cos(2t)\qquad(t>0)\ .$$