问题 解答题
先化简,再求值:
1
2x
-
1
x+y
(x2-y2+
x+y
2x
)
,其中x=1,y=3.
答案

1
2x
-
1
x+y
(x2-y2+
x+y
2x
)

=

1
2x
-
1
x+y
[(x+y)(x-y)+
x+y
2x
]

=

1
2x
-(x-y)-
1
2x

=y-x,

∴当x=1,y=3时,原式=3-1=2.

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