已知x+y-3=0 求(x²-y²)²-8(x²+y²)

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已知x+y-3=0 求(x²-y²)²-8(x²+y²)
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(x²-y²)²-8(x²+y²)
=((x+y)(x-y))2-8(x2+y2) ( x+y-3=0所以x+y=3代入）
=(3(x-y))2-8(x2+y2)
=9(x2-2xy+y2)-8(x2+y2)
=9x2-18xy+9y2-8x2-8y2
=x2-18xy+y2

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