\[
\boxed{
{\frac {\rm d}{{\rm d}x}}y \left( x \right) + \left( {\frac {\rm d}{{\rm d}x}}f \left( x \right)  \right) y \left( x \right) -f \left( x \right) {\frac {\rm d}{{\rm d}x}}f \left( x \right) =0}
\]