\[ \boxed { \left ( \left ( {\frac {\rm d}{{\rm d}x}}y \left ( x \right ) \right ) ^{2}+ \left ( y \left ( x \right ) \right ) ^{2} \right ) \left ( \cos \left ( x \right ) \right ) ^{4}-{a}^{2}=0} \]
Mathematica: cpu = 56.015113 (sec), leaf count = 28 \[ \text {DSolve}\left [\cos ^4(x) \left (y'(x)^2+y(x)^2\right )-a^2=0,y(x),x\right ] \]
Maple: cpu = 5.163 (sec), leaf count = 0 \[ \text {could not solve} \]