a=2003,
b=2004,
c=2005
a-b=-1
b-c=-1
c-a=2
a^2+b^2+c^2-ab-bc-ac
=(2a^2+2b^2+2c^2-2ab-2bc-2ac)/2
=[(a^2-2ab+b^2)+(b^2-2bc+c^2)+(c^2-2ca+a^2)]/2
=[(a-b)^2+(b-c)^2+(c-a)^2]/2
=[(-1)^2+(-1)^2+2^2]/2
=3
a=2003,
b=2004,
c=2005
a-b=-1
b-c=-1
c-a=2
a^2+b^2+c^2-ab-bc-ac
=(2a^2+2b^2+2c^2-2ab-2bc-2ac)/2
=[(a^2-2ab+b^2)+(b^2-2bc+c^2)+(c^2-2ca+a^2)]/2
=[(a-b)^2+(b-c)^2+(c-a)^2]/2
=[(-1)^2+(-1)^2+2^2]/2
=3