Q5 of 176 Page 8

If the sum to first n terms of an A.P. is , find its 25th term.


Taking n = 1, we get




⇒ S1 = 4


⇒ a1 = 4


Taking n = 2, we get




⇒ S2 = 11


∴ a2 = S2 – S1 = 11 – 4 = 7


Taking n = 3, we get




⇒ S3 = 21


∴ a3 = S3 – S2 = 21 – 11 = 10


So, a = 4,


d = a2 – a1 = 7 – 4 = 3


Now, we have to find the 25th term


an = a + (n – 1)d


a25 = 4 + (25 – 1)3


a25 = 4 + 24 × 3


a25 = 4 + 72


a25 = 76


Hence, the 25th term is 76.


More from this chapter

All 176 →
3

The sum of n terms of an A.P. is 3n2+ 5n. Find the A.P. Hence, find its 16th term.

 4

If the sum of the first n terms of an A.P. is given by Sn = (3n2- n), find its

(i) first term (ii) common difference


(iii) nth term.

 6

If the nth term of an A.P. is (2n + 1), find the sum of first n terms of the A.P.

 7

If the nth term of an A.P. is 9 — 5n, find the sum to first 15 terms.