Áp dụng tính chất dãy tỷ số bằng nhau, ta có:
\(\frac{a+b+c}{a+b-c}=\frac{a-b+c}{a-b-c}=\frac{\left(a+b+c\right)-\left(a-b+c\right)}{\left(a+b-c\right)-\left(a-b-c\right)}=\frac{a+b+c-a+b-c}{a+b-c-a+b+c}=\frac{2b}{2b}=1\)
\(\Rightarrow\frac{a+b+c}{a+b-c}=1\)
\(\Rightarrow a+b+c=a+b-c\)
\(\Rightarrow c=-c\)
\(\Rightarrow c=0\)( đpcm )