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)[(a²+2ab+b²)+(b²+2bc+c²)+(a&#...