问已知x²+y²=7,xy=－2,求5x²-3xy - 知识问答

问已知x²+y²=7,xy=－2,求5x²-3xy-4y²-11xy-7x&su

1个回答

x^2+y^2=7,xy=-2
5x^2-3xy-4y^2-11xy-7x^2+2y^2
=-2x^2-14xy-2y^2
=-2(x^2+y^2)-14xy
=-2*7-14*(-2)
=-14+28
=14

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