=[a+(b一C)][a一(b一c)]一[a一(b+C)][a+(b+C)]=a²一(b一C)²一[a²一(b+c)²]=a²一b²+2bc一c²一(a²一b²一2bc一C²)=a²一b²一C²+2bc一a²+b²+2bc+c²=4bc