@PaulGessler So do you mean that it should be like that? \begin{multline} y'=y+4 \pi \cos(4 \pi t)y \Rightarrow y'=(1+ 4 \pi \cos(4 \pi t))y \cap \Rightarrow \frac{dy}{dt}=(1+ 4 \pi \cos(4 \pi t))y \cap \Rightarrow \frac{dy}{y}=(1+ 4 \pi \cos(4 \pi t)) dt \end{multline}
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