Mathematics
Let \(a_{1},a_{2},\ldots,a_{21}\) be an AP such that
\(\sum_{n = 1}^{20}\mspace{2mu}\frac{1}{a_{n}a_{n + 1}} = \frac{4}{9}\).
If the sum of this AP is 189 , then \(a_{6}a_{16}\) is equal to
A
57
B
48
C
36
D
72
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