Q1 of 176 Page 8

The sum of n terms of an A.P. is . Find its 20th term.


Taking n = 1, we get




⇒ S1 = 4


⇒ a1 = 4


Taking n = 2, we get




⇒ S2 = 13


∴ a2 = S2 – S1 = 13 – 4 = 9


Taking n = 3, we get




⇒ S3 = 27


∴ a3 = S3 – S2 = 27 – 13 = 14


So, a = 4,


d = a2 – a1 = 9 – 4 = 5


Now, we have to find the 20th term


an = a + (n – 1)d


a20 = 4 + (20 – 1)5


a20 = 4 + 19 × 5


a20 = 4 + 95


a20 = 99


Hence, the 20th term is 99.


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