(x-y)^2-(2x-y)^2 4x^2-nxy+9y^2
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(x-y)^2-(2x-y)^24x^2-nxy+9y^2(x-y)^2-(2x-y)^24x^2-nxy+9y^2(x-y)^2-(2x-y)^24x^2-nxy+9y^2(x-y)^2-(2x-y
(x-y)^2-(2x-y)^2 4x^2-nxy+9y^2
(x-y)^2-(2x-y)^2 4x^2-nxy+9y^2
(x-y)^2-(2x-y)^2 4x^2-nxy+9y^2
(x-y)^2-(2x-y)^2
=[(x-y)-(2x-y)][(x-y)+(2x-y)]
=(-x)(3x-2y)
=-x(3x-2y)
4x^2-nxy+9y^2 (n=12 )
4x^2-12xy+9y^2
=(2x-3y)^2
(x-y)^2-(2x-y)^2=-x（3x-2y）
4x^2-nxy+9y^2=(2x-3y)^2