问题 选择题
已知
1
x
-
1
y
=2
,则
5x+xy-5y
x-xy-y
等于(  )
A.
1
3
B.-
1
3
C.-3D.3
答案

1
x
-
1
y
=
y-x
xy
=2,即y-x=2xy,

∴x-y=-2xy,

5x+xy-5y
x-xy-y
=
5(x-y)+xy
(x-y)-xy
=
-10xy+xy
-2xy-xy
=
-9xy
-3xy
=3.

故选D

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