aa+bb-2a+2b+2=(a^2-2a+1)+(b^2-2b+1)=(a-1)^2+(b+1)^2=0
a=1.b=-1
a^2004+b^2005=1-1=0
aa+bb=5 a+b=3 则ab=
aa+bb=(a+b)^2-2ab=9-2ab=5,ab=2
aa+bb+cc-ab-bc-ac=0.5*(a^2-2ab+b^2+a^2-2ac+c^2+b^2-2bc+c^2)=0.5*[(a-b)^2+(a-c)^2+(b-c)^2]