Q24 of 24 Page 115

In an A.P the sum of its first n term is 6n–n2. Find its 25th term.

Given, Sum of n terms:


Sn = 6n – n2


25th term = Sum of 25 terms – Sum of 24 terms


⇒ a25 = S25 – S24


⇒ a25 = 6(25) – 25(25) – (6(24) – (24)2)


⇒ a25 = 150 – 625 – 144 + 576


⇒ a25 = –43


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