a,b为负数,a比b小,c为正数则
b-a>0,a-c=0时,
丨b-a丨+丨b+c丨-丨a-c丨
=b-a+b+c-(c-a)
=2b
当b+c< =0时
丨b-a丨+丨b+c丨-丨a-c丨
=b-a-b-c-(c-a)
=-2c