初一数学题啊....急

来源：百度知道 编辑：UC知道 时间：2024/06/16 00:16:36
已知x^2+y^2=7,xy=-1,求代数式5x^2-(3xy+4y^2)-(11xy-2y^2)-7x^2的值.

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已知x^2+y^2=7,xy=-1,求代数式5x^2-(3xy+4y^2)-(11xy-2y^2)-7x^2的值.
已知x^2+y^2=7,xy=-1,
5x^2-(3xy+4y^2)-(11xy-2y^2)-7x^2

=5x^2-3xy-4y^2-11xy+2y^2-7x^2
=-2x^2-14xy-2y^2
=-2(x^2+y^2)-14xy
=-2*7-14*(-1)
=-14+14
=0

0

原式=5x^2-3xy-4y^2-11xy+2y^2-7x^2
=-2x^2-2y^2-14xy
=-2(x^2+y^2)+14
=-14+14=0

0
原式=5x^2-3xy-4y^2-11xy+2y^2-7x^2
=5x^2-7x^2-4y^2+2y^2-3xy-11xy
=-2x^2-2y^2-14xy=-14+14=0