(x+y)^2+9(x-y)^2-6(x^2-y^2)

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(x+y)^2+9(x-y)^2-6(x^2-y^2)
(x+y)^2+9(x-y)^2-6(x^2-y^2)

(x+y)^2+9(x-y)^2-6(x^2-y^2)
原式=x^2+2xy+y^2+9x^2-18xy+9y^2-6x^2+6y^2=4x^2-16xy+16y^2

(x+y)^2+9(x-y)^2-6(x^2-y^2)
=(x+y)²-2×(x+y)×3(x-y)+[3(x-y)]²
=[(x+y)-3(x-y)]²
=(4y-2x)²
=4(x-2y)²

(x+y)^2+9(x-y)^2-6(x^2-y^2)
=(x+y-3x+3y)^2
=(-2x+4y)^2
=4(x-2y)^2

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