当x＜1时,
丨x-1丨+丨x-2丨
=1-x+2-x
=3-2x
当1≤x≤2时
丨x-1丨+丨x-2丨
=x-1+2-x
=1
当x＞2时
丨x-1丨+丨x-2丨
=x-1+x-2
=2x-3