(a-b-c)²
=a²﹢b²﹢c²﹣2ab﹣2ac﹢2bc（a-b+c)²=a²﹢b²﹢c²﹣2ab﹢2ac﹣2bc（a+b-c)²=a²﹢b²﹢c²﹢2ab﹣2ac﹣2bc（a+b+c)²=a²﹢b²﹢c²﹢2ab﹢2ac﹢2bc