即(x-2y)²=0
x-2y=0
所以x=2y
所以原式=(2x²+2xy-xy-y²)/(4x²-4xy+y²)
=(2x²+xy-y²)/(4x²-4xy+y²)
=(2x-y)(x+y)/(2x-y)²
=(x+y)/(2x-y)
=(2y+y)/(4y-y)
=3y/3y
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