\[
\boxed{
\sqrt {a \left( {\frac {{\rm d}^{2}}{{\rm d}{x}^{2}}}y \left( x \right)  \right) ^{2}+b \left( {\frac {\rm d}{{\rm d}x}}y \left( x \right)  \right) ^{2}}+cy \left( x \right) {\frac {{\rm d}^{2}}{{\rm d}{x}^{2}}}y \left( x \right) +d \left( {\frac {\rm d}{{\rm d}x}}y \left( x \right)  \right) ^{2}=0}
\]