a,b,c是三角形的三边长
b+c>a a-(b+c)<0
a+c>b b-(a+c)<0
a+b>c c-(a+b)<0
|a-b-c|+|b-a-c|+|c-a-b|
=|a-(b+c)|+|b-(a+c)|+|c-(a+b)|
=b+c-a+a+c-b+a+b-c
=a+b+c
a,b,c是三角形的三边长
b+c>a a-(b+c)<0
a+c>b b-(a+c)<0
a+b>c c-(a+b)<0
|a-b-c|+|b-a-c|+|c-a-b|
=|a-(b+c)|+|b-(a+c)|+|c-(a+b)|
=b+c-a+a+c-b+a+b-c
=a+b+c