化简求值:若x^2+y^2-x+4y+17/4=0.求(2x+y)(2x-y)-(2x-y)^2的值.

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x^2+y^2-x+4y+17/4=0
(x²-x+¼)+(y²+4y+4)=0
(x-½)²+(y+2)²=0
x-1/2=0,y+2=0
x=1/2,y=-2
(2x+y)(2x-y)-(2x-y)^2
=(2x-y)(2x+y-2x+y)
=2y(2x-y)
=-4×3
=-12