Let \(a_1, a_2, \ldots \ldots, a_{21}\) be an A.P. such that \(\sum_{n=1}^{20} \frac{1}{a_n a_{n+1}}=\frac{4}{9}\). If t…

Let \(a_1, a_2, \ldots \ldots, a_{21}\) be an A.P. such that \(\sum_{n=1}^{20} \frac{1}{a_n a_{n+1}}=\frac{4}{9}\). If the sum of this A.P. is 189, then \(a_6 a_{16}\) is equal to:

[JEE Main 2021, 1 Sep (Shift 2)]