=(x2-xy)+(4x-4y)(分成两组)
=x(x-y)+4(x-y)(直接提公因式)
=(x-y)(x+4) (2)a2-b2-c2+2bc
=a2-(b2+c2-2bc)(分成两组)
=a2-(b-c)2(直接运用公式)=(a+b-c)(a-b+c)
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