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Let a1, a2, …, a21 be an AP such that s...

Let `a_1, a_2, …, a_21 ` be an AP such that `sum_(n=1)^20 1/(a_n a_(n+1)) = 4/9`. if the sum of this AP is 189, then `a_6a_16` is equal to :

A

36

B

48

C

72

D

57

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