(別証)
 kk = L(2-L) とおく。
 L = 1 - √(1-kk) ≧ 0,
 1^2 - (ks)^2 = 1^2 - L(2-L)s^2 = (1-Lss)^2 + (1-ss)(Ls)^2,
 s = sinφ,