Which term of the AP 2, −1, −4, −7, ... is −40?
(a) 8th
(b) 11th
(c) 14th
(d) 15th

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Solution

(d) 15^th

Here, a = 2, d = (−1 − 2) = −3
Let the n^th term be −40.
Then Tn = 40
⇒ a + (n − 1)d = −40
⇒2 + (n − 1) ⨯ (−3) = −40
⇒ 3n = 45
⇒ n = 15

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