Q7 of 176 Page 8

Which term of the A.P. 5, 9, 13, 17, ... is 81?

Here, a = 5, d = 9 – 5 = 4 and an = 81


To find : n


We have,


an = a + (n – 1)d


⇒ 81 = 5 + (n – 1) × 4


⇒ 81 = 5 + 4n – 4


⇒ 81 = 4n + 1


⇒ 80 = 4n


⇒ n = 20


Therefore, the 20th term of the given AP is 81.


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