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