1/x-1/y=(y-x)/xy=3
y-x=3xy
x-y=-3xy
原式=[5(x-y)+xy]/[(x-y)-xy]
=[5(-3xy)+xy]/[(-3xy)-xy]
=-14xy/(-4xy)
=7/2
1/x-1/y=3 5x+xy-5y/x-xy-y=
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