Q. 7 A5.0( 4 Votes )

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

Answer :

Given: an = 9 – 5n


Taking n = 1,


a1 = 9 – 5(1) = 9 – 5 = 4


Taking n = 2,


a2 = 9 – 5(2) = 9 – 10 = -1


Taking n = 3,


a3 = 9 – 5(3) = 9 – 15 = -6


Therefore the series is 4, -1, -6, …


So, a = 4, d = a2 – a1 = -1 – 4 = -5


Now, we have to find the sum of the first 15 terms of the AP






S15 = 15 × (-31)


S15 = -465


Hence, the sum of 15 terms is -465.


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