∵a-b=-1,b-c=-1∴a-c=a-b+b-c=-1-1=-2a²+b²+c²-ab-bc-ac=½﹙2a²+2b²+2c²-2ab-2ac-2bc)=½[﹙a²-2ab+b²﹚+(a²-2ac+c²)+(b²-2bc+c²)]=½[...