a2+b2+c2-ab-ac-bc= 1/2 [ (a2 -2ab +b2) + (a2 - 2ab + c2 ) + (b2 - 2bc + c2) ]= 1/2 [ (a-b)2 + (a-c)2 + (b-c)2 ]= 1/2 [ (1)2 + (-1)2 + (-2)2 ]= 1/2 ( 1 + 1 + 4 )= 3