Let be an AP such that

MATHEMATICS · Single correct · CBSE · Mathematics · Arithmetic Progressions · Arithmetic Progressions

Problem

Let

a_1, a_2, \dots, a_{21}

be an AP such that

\sum_{n=1}^{20} \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) 72 (C) 48 (D) 36