x-3xy+y/x+2xy+y
=x+2xy+y-5xy/x+2xy+y
=1-5xy/x+2xy+y
=1-5/1/y+2+1/x
=1-5/3+2
=1-5/5
=0
1/x+1/y=(x+y)/xy=3
x+y=3xy
所以原式=[(x+y)-3xy]/[(x+y)+2xy]
=(3xy-3xy)/(3xy+2xy)
=0/5xy
=0
1/x+1/y=3 则 x-3xy+y=0
x-3xy+y/x+2xy+y=(x-3xy+y)/(x+2xy+y)=0
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