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Mathematics
Mathematics
Problem
Let
a
1
,
a
2
,
…
,
a
21
a_1, a_2, \dots, a_{21}
a
1
,
a
2
,
…
,
a
21
be an AP such that
∑
n
=
1
20
1
a
n
a
n
+
1
=
4
9
\sum_{n=1}^{20} \frac{1}{a_n a_{n+1}} = \frac{4}{9}
∑
n
=
1
20
a
n
a
n
+
1
1
=
9
4
. If the sum of this AP is 189, then
a
6
a
16
a_6 a_{16}
a
6
a
16
is equal to: (A) 57 (B) 72 (C) 48 (D) 36
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