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若x-y=4,求(xx+yy)÷2-xy的值

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若x-y=4,求(xx+yy)÷2-xy的值
(x^2+y^2)/2-xy
=(x^2+y^2-2xy+2xy)/2-xy
=[(x-y)^2+2xy]/2-xy
=(x-y)^2/2+xy-xy
=(x-y)^2/2
=4^2/2
=8
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