已知 1 / X(n-1) + 1 / X(n+1) = 2 / Xn可知 {1/Xn}为等差数列设 An = 1/Xn A1=1 公差d=1/X2 - 1/X1 = 1/2所以 An = A1 + (n-1)d = 1 + 1/2(n-1) = 1/2(n+1)所以 Xn = 2/(n+1) (n>=2)