（a-b-c)(-a＋b-c) =（a-b-c)[-(a-b+c)] =-（a-b-c)(a-b+c) =-[(a-b)^2-c^2] =c^2-(a-b)^2 =c^2-a^2-b^2+2ab 不好意思,少打了一个括号
（a-b-c)(-a＋b-c) =-((a-b)-c)((a-b)+c) =-(a-b)^2-c^2 =-(a^2-2ab+b^2-c^2) =c^2-a^2-b^2+2ab
(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^2+2ab-b^2+c^2
（a-b-c)(-a＋b-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^2+2ab-b^2+c^2
=[-c+(a-b)][-c-(a-b)] =(-c)^2-(a-b)^2 =c^2-(a-b)^2
（a-b-c)(-a＋b-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^2+2ab-b^2+c^2
(a-b-c)(-a+b-c) =c^2-(a-b)^2 =c^2-a^2-b^2+2ab
（a-b-c)(-a＋b-c) =-((a-b)-c)((a-b)+c) =-(a-b)^2-c^2 =-(a^2-2ab+b^2-c^2) =c^2-a^2-b^2+2ab
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