问题 解答题
已知:x2+y2=2,xy=-
1
2
,求代数式:(2x2-y2-3xy)-(x2-2y2+xy)的值.
答案

(2x2-y2-3xy)-(x2-2y2+xy)

=2x2-y2-3xy-x2+2y2-xy

=x2+y2-4xy,

x2+y2=2,xy=-

1
2
时,原式=2-4×
1
4
=2-2=0.

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