a^2+b^2+c ^2-ab-ac-bc
＝(2a^2+2b^2+2c ^2-2ab-2ac-2bc）/2
＝〔(a^2-2ab+b^2)+(a^2-2ac+c^2)+(b^2-2bc+b2）〕/2
=〔(a-b)2+(a-c)^2+(b-c)^2〕/2
=〔(2011x+2010-2011x-2011)2+(2011x+2010-2011x-2012)^2+(2011x+2011-2011x-2012)^2〕/2
=(1+4+1)/2
=3