重点：分类讨论 || X-1 | -3 | + | 3X +1 | 时x≥4，则：X-4 + 3×+1 = 4X-3 当-2≤x≤-1/3则：| 1-X-3 | -3x-1 = 2 + X-3X-1 = 1-2倍时。 x≤-2是： - × - 2-3倍，1 = -4x-3 时1≤x≤4，则| X-1-3 | + 3×+ 1 = 4-X + 3×+1 = 5-2x
3（x+1）/2x+10x/(x+1)+2
