\[
\boxed{
{\frac {{\rm d}^{2}}{{\rm d}{x}^{2}}}y \left( x \right) =-{\frac { \left( -a \left( \cos \left( x \right)  \right) ^{2} \left( \sin \left( x \right)  \right) ^{2}-m \left( m-1 \right)  \left( \sin \left( x \right)  \right) ^{2}-n \left( n-1 \right)  \left( \cos \left( x \right)  \right) ^{2} \right) y \left( x \right) }{ \left( \cos \left( x \right)  \right) ^{2} \left( \sin \left( x \right)  \right) ^{2}}}=0}
\]