已知:x²+xy=12,xy+y²=15,求2x²-2y²的值,

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已知:x²+xy=12,xy+y²=15,求2x²-2y²的值,
已知:x²+xy=12,xy+y²=15,求2x²-2y²的值,

已知:x²+xy=12,xy+y²=15,求2x²-2y²的值,
两式相减
x²+xy-xy-y²=12-15
x²-y²=-3
2x²-2y²=-6

2x²-2y²
=2(x²-y²)
=2(x²+xy-xy-y²)
=2[(x²+xy)-(xy+y²)]
=2[12-15]
=2×(-3)
= -6