cot−1xy+1x−y+cot−1yz+1y−z+cot−1xz+1z−x is equal to
1
-1
0
π
cot−1xy+1x−y+cot−1yz+1y−z+cot−1xz+1z−x =tan−1x−y1+xy+tan−1y−z1+yz+cot−1xz+1z−x =(tan−1x−tan−1y)+(tan−1y−tan−1z)+(tan−1z−tan−1x)=0