3 / (1 + x) = (x^2 - x + 1) / (1+ x)
两边同时乘以(1 + x)得 x^2 - x + 1 = 3
整理得 x^2 - x - 2 = 0
分解得 (x - 2)(x + 1) = 0
所以 x = 2.
(x = -1 时分母为零,舍去)