题目内容
(2012•北碚区模拟)若
+
+
+…+
的值为
,则正整数n的值是( )
1 |
1×3 |
1 |
3×5 |
1 |
5×7 |
1 |
(2n-1)(2n+1) |
17 |
35 |
分析:首先根据
=
(
-
)把已知的代数式进行化简,然后解方程即可求得n的值.
1 |
(2n-1)(2n+1) |
1 |
2 |
1 |
2n-1 |
1 |
2n+1 |
解答:解:原式=
(1-
)+
(
-
)+…+
(
-
)
=
(1-
+
-
+…+
-
)
=
(1-
)
=
×
=
=
.
解得:n=17.
1 |
2 |
1 |
3 |
1 |
2 |
1 |
3 |
1 |
5 |
1 |
2 |
1 |
2n-1 |
1 |
2n+1 |
=
1 |
2 |
1 |
3 |
1 |
3 |
1 |
5 |
1 |
2n-1 |
1 |
2n+1 |
=
1 |
2 |
1 |
2n+1 |
=
1 |
2 |
2n |
2n+1 |
=
n |
2n+1 |
=
17 |
35 |
解得:n=17.
点评:本题考查了分式的化简求值,正确理解
=
(
-
)是关键.
1 |
(2n-1)(2n+1) |
1 |
2 |
1 |
2n-1 |
1 |
2n+1 |
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