已知(x+y)²=18,(x-y)²=6,求:①x²+y²的值;②xy的值.
3个回答
x²+y²
=[(x+y)²+(x-y)²]/2
=(18+6)/2
=24/2
=12
=12
xy=[(x+y)²-(x-y)²]/4
=(18-6)/4
=12/4
=3
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