3,4题帮下忙,

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3,4题帮下忙,
3,4题帮下忙,

3,4题帮下忙,
3.（1）（-2x+5）^2-（-2x-5）^2
=4x^2-20x+25-（4x^2+20x+25)
=-20x
(2)(x-y)(x^2-y^2)(x+y)
=(x^2-y^2）^2
=x^4-2(xy)^2+y^4
4.将两式相加得：
2(x^2+y^2)=10
x^2+y^2=5
将x^2+y^2=5代入其中一式
解得：xy=1

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