∵ 2(a^2+b^2+c^2-ab-bc-ac) =2a^2+2b^2+2c^2-2ab-2bc-2ac =(a^2-2ab+b^2)+(b^2-2bc+c^2)+(a^2-2ac+c^2) =(a-b)^2+(b-c)^2+(a-c)^2 =(2009-2010)^2+(2010-2011)^2+(2009-2011)^2 =1+1+4=6 ∴a^2+b^2+c^2-ab-bc-ac=6/...