Question

已知x+y=2,xy=-3,求多项式1/2(x²+y²)-xy的值

Answer 1

(x+y)²=4

x²+2xy+y²=4

x²+y²-6=4

x²+y²=10

1/2(x²+y²)-xy

=1/2x10-(-3)

=5+3

=8

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