I was reading Particular Integral.
Following is done in my book. I can not understand how the third line came from second line.
$\frac{1}{D^2+a^2} \sin ax = \frac{1}{D^2+a^2}(\text{imaginary part of } \cos ax + \iota \sin ax) = \text{imaginary part of } (\frac{1}{D^2+a^2} \cos ax + \iota \frac{1}{D^2+a^2} \sin ax ) $
Here $D$ is $\frac{d}{dx}$.
Can anyone please help me to understand ?