a²+b²+c²+ab+bc+ac=1/2(2a²+2b²+2c²+2ab+2bc+2ac)=1/2[(a²+2ab+b²)+(b²+2bc+c²)+(a²+2ac+c²)]=1/2[(a+b)²+(b+c)²+(a+c)²]=1/2(a+b)
...