1

2x

-

1

x+y

（

x+y

2x

-x-y-1）

=

1

2x

-

1

x+y

[

x+y

2x

-（x+y+1）]

=

1

2x

-

1

x+y

•

x+y

2x

+

x+y+1

x+y

=

1

2x

-

1

2x

+1+

1

x+y

=1+

1

x+y

，

当x=1，y=0时，原式=1+

1

1+0

=2．