Question 731473
x^2 + y^2 + z^2 -xy - yz- zx  = 1/2{(x-y)^2 + (y-z)^2 + (z-x)^2 } 

= 1/2{(97-98)^2 + (98-99)^2 + (99-97)^2 } = 1/2 ( 1 + 1 + 4 ) = 6/2 = 3