>>541
AB = 2a,
CD = 2b,
a ≧ b > 0,
とおくと
 K = 4a∫[0,π/2] √{1 -(k・sinφ)^2}dφ = 4a E(k), k=√(1-bb/aa),
 L = 2πa,
 M = 2πb,

 K ≧ π(a+b) = (L+M)/2.