a,b,c是三角形ABC的三边长则 b+c>a a+c>b a+b>cb+c-a|+|b-c-a|-|c-a-b|=b+c-a+a+c-b+a+b-c=a+b+c
|b+c-a|+|b-c-a|-|c-a-b|=b+c-a-b+c+a-c+a+b=a+b+c
b+c>a a+c>b a+b>c b+c-a|+|b-c-a|-|c-a-b| =b+c-a+a+c-b+a+b-c =a+b+c
