\(S=-\left(a-b-c\right)+\left(-c+b+a\right)-\left(a+b\right)\)
\(=-a+b+c-c+b+a-a-b\)
\(=-a\)
\(\Rightarrow\left|S\right|=\left|-a\right|=a\)
Vậy ................
ta có : S=-(a-b-c)+(-c+b+a)-(a+b)
=-a+b+c-c+b+a-a-b
=-a\(\Rightarrow\)\(\left|S\right|\)=\(\left|-a\right|\)=a