x＜-1时：
= |1-x-2| -(x+1)
= |-x-1| - (x+1)
= -(x+1)-(x+1)
= -2x-2
-1≤x≤1时：
= |1-x-2| +(x+1)
= |-x-1| + (x+1)
= (x+1)+(x+1)
= 2x+2
1＜x＜3时：
= |x-1-2|+(x+1)
= |x-3| +(x-1)
= 3-x+x-1
= 2
x≥3时：
= |x-1-2|+(x+1)
= |x-3| +(x-1)
= x-3+x-1
= 2x-4
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