(a-c)(b-c)=ab-bc-ac+c^2=c^2-ba-ca ≤0 1 ≤ba+ca
|a+b-c|^2=a^2+b^2+c^2+2ab-2bc-2ac=a^2+b^2+c^2-2bc-2ac≤1+1+1-2=1