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.16 B.17 C.18 D.19

问题描述:

若 

1
1×3
+
1
3×5
+
1
5×7
+…+
1
(2n−1)(2n+1)
的值为
17
35
,则正整数n的值是(  )
A. 16
B. 17
C. 18
D. 19

原式=

1
2
(1-
1
3
)+
1
2
1
3
-
1
5
)+…+
1
2
1
2n−1
-
1
2n+1

=
1
2
(1-
1
3
+
1
3
-
1
5
+…+
1
2n−1
-
1
2n+1

=
1
2
(1-
1
2n+1

=
1
2
×
2n
2n+1

=
n
2n+1

=
17
35

解得:n=17.