9（x-y)平方+12(x平方-y平方）+4（x+y)平方=9(x^2-2xy+y^2)+12x^2-12y^2+4(x^2+2xy+y^2)=9x^2-18xy+18y^2+12x^2-12y^2+4x^2+8xy+4y^2=25x^2-10xy+10y^2
9（x�0�5-2xy+y�0�5）+12（x�0�5-y�0�5）+4（x�0�5+2xy+y�0�5）= （9+12+4）x�0�5+（-18+8）xy+（9-12+4）y�0�5= 25x�0�5-10xy+y�0�5
=9（x-y)�0�5+12（x+y)(x-y)+4(x+y)=[3(x-y)+2(x+y)]�0�5=（5x-y)�0�5
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