∑ 11+xa-b+xa-c=
1
-1
0
None of these
∑ 11+xa-b+xa-c=∑ xb+cxb+c+xc+a+xa+b =1xb+c+xc+a+xa+b∑ xb+c =1xb+c+xc+a+xa+bxb+c+xc+a+xa+b=1.