已知x+y=7 xy=2 求2x²+2y²的值
问题描述：
已知x+y=7 xy=2 求2x²+2y²的值
答
2x²+2y²
=2(x²+y²)
=2[(x²+2xy+y²)-2xy]
=2[(x+y)²-2xy]
=2(x+y)²-4xy
=2×7²-4×2
=98-8
=90
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