\left. \begin{gathered}
 y = C_1 \cos x + C_2 \sin x \Rightarrow y_0 = C_1 \hfill \\
 y' = - C_1 \sin x + C_2 \cos x \Rightarrow y_1 = C_2 \hfill \\ 
\end{gathered} \right\} \Rightarrow y = y_0 \cos x + y_1 \sin x