Q. 7 B5.0( 5 Votes )

Find the sum of first 25 terms of an A.P. whose nth term is 1 — 4n.

Answer :

Given: an = 1 – 4n


Taking n = 1,


a1 = 1 – 4(1) = 1 – 4 = -3


Taking n = 2,


a2 = 1 – 4(2) = 1 – 8 = -7


Taking n = 3,


a3 = 1 – 4(3) = 1 – 12 = -11


Therefore the series is -3, -7, -11, …


So, a = -3, d = a2 – a1 = -7 – (-3) = -7 + 3 = -4


Now, we have to find the sum of the first 25 terms of the AP






S25 = 25 × (-51)


S25 = -1275


Hence, the sum of 25 terms is -1275.


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