>>79
簡単な計算により
　F[2n] = 2 F[n+1] F[n] - F[n]^2
よって
　F[2n] ≧ 2 F[n]^2 - F[n]^2 = F[n]^2
　F[2n] ≦ F[n+1]^2 - (F[n-1] - F[n])^2 ≦ F[n+1]^2