f(x)
= (1 - 3x)/(x + 1)
= (-3x - 3 + 4)/(x + 1)
= -3 + 4/(x + 1)
因为 4/(x + 1) ≠ 0
所以 -3 + 4/(x + 1) ≠ -3
所以值域是 (-∞ ,-3)U(-3 ,+∞)