Which term of the AP: 21,18,15,...... is -81?

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Solution

The series is $21, 18, 15, . . .$

$a = 21, d = - 3$

$a_{n} = - 81 a_{n} = a + (n - 1) d - 81 = 21 + (n - 1) (- 3) - 81 - 21 = - 3 (n - 1) 102 = 3 (n - 1) n - 1 = 34 n = 34 + 1 = 35$

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