Question:
Which term of the AP 21, 42, 63, 84, ... is 210?
(a) 9th
(b) 10th
(c) 11th
(d) 12th
Solution:
(b) 10th
Here, a = 21 and d = (42 - 21) = 21
Let 210 be the nth term of the given AP.
Then Tn = 210
⇒ a + (n - 1)d = 210
⇒ 21 + (n - 1) ⨯ 21= 210
⇒ 21n = 210
⇒ n = 10
Hence, 210 is the 10th term of the AP.
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