a,c＜0,b＞0,a的绝对值大于b,c的就对值大于b和a
丨a-c丨-丨a-b丨-丨b-c丨十丨2a丨
=a-c-(b-a)-(b-c)-2a
=a-c-b+a-b+c-2a
=-2b