(a+b+c)(a十b-c)=(a十b)^一c^=a^十2ab十b^一c^
(a＋b＋c)(a＋b－c)=[(a+b)+c][(a+b)-c]=(a+b)^2-c^2=a^2+2ab+b^2-c^2。
(a-b+c)(a+b-c)=[a-(b-c)]*[a+(b-c)]=a²-(b-c)²=a²-(b²-2bc+c²)=a²-b²穿沪扁疚壮狡憋挟铂锚-c²+2bc
