(a-b+c)(a+b-c)=[a-(b-c)]*[a+(b-c)]=a�0�5-(b-c)�0�5=a�0�5-(b�0�5-2bc+c�0�5)=a�0�5-b�0�5-c�0�5+2bc
（a-b+c)*(a+b-c)=a*a-b*b+c*(-c)=2a-0-0=2a
