若 11×3+13×5+15×7+…+1(2n−1)(2n+1)的值为1735,则正整数n的值是( )A. 16B. 17C. 18D. 19
问题描述:
若
+1 1×3
+1 3×5
+…+1 5×7
的值为1 (2n−1)(2n+1)
,则正整数n的值是( )17 35
A. 16
B. 17
C. 18
D. 19
答
知识点:本题考查了分式的化简求值,正确理解
=
(
-
)是关键.
原式=
(1-1 2
)+1 3
(1 2
-1 3
)+…+1 5
(1 2
-1 2n−1
)1 2n+1
=
(1-1 2
+1 3
-1 3
+…+1 5
-1 2n−1
)1 2n+1
=
(1-1 2
)1 2n+1
=
×1 2
2n 2n+1
=
n 2n+1
=
.17 35
解得:n=17.
答案解析:首先根据
=1 (2n−1)(2n+1)
(1 2
-1 2n−1
)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
考试点:分式的加减法.
知识点:本题考查了分式的化简求值,正确理解
1 |
(2n−1)(2n+1) |
1 |
2 |
1 |
2n−1 |
1 |
2n+1 |