\[
\boxed{
 \left(  \left( {\frac {\rm d}{{\rm d}x}}y \left( x \right)  \right) ^{2}+ \left( y \left( x \right)  \right) ^{2} \right)  \left( \cos \left( x \right)  \right) ^{4}-{a}^{2}=0}
\]