x-y=3=>(x-y)^2=97-x+y=7-(x-y)=4故=-5现在的初中生都会用网络了吗，真是神奇...
原式=7-(x-y)-(x-y)^2=7-3-9=-5要记住（a-b)^2=(b-a)^2
7-x+y-(y-x)^2=7-(x-y)-(x-y)^2=7-3-3^2=7-3-9=-5
7-(x-y)-(x-y)^2=-5
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