[(x-y)^2 +(x+y) (x-y)]÷2x
=[（x-y）(x-y+x+y)]÷2x
=(x-y)×2x÷2x
=x-y
∵x=9,y=-1999
∴原式=9-(-1999)=2008
[(x-y)^2 +(x+y) (x-y)]÷2x
=(x-y)[(x-y)+(x+y)]/2x
=(x-y)*2x/2x
=x-y
=2008
先打开小括号[x^2-2xy+y^2+x^2-y^2]/2x
=[2x^2-2xy]/2x
=x-y=9-(-1999)=2008
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