已知x+y=2,xy=-5求
（1)1/x+1/y的值
=（x+y）/(xy)
=2/(-5)
=-2/5;
（2）求1/x²+1/y²的值
=(x²+y²)/(x²y²)
=[(x+y)²-2xy]/(x²y²)
=[2²+10]/25
=14/25;
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