a^2+b^2+c^2+ab+bc-ca=2(a^2+b^2+c^2+ab+bc-ca)/2
=(a^2+b^2+2ab+b^2 +c^2 +2bc +a^2+ c^2-2ca)/2
=[(a+b)^2+(b+c)^2+(a-c)^2]/2
=[1+4+1]/2
=3