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Question
In an A.P., the sum of its first n terms is 6n – n². Find is 25th term.
Solution
Sn = 6n – n²
T25 = ?
S(n–1) = 6(n – 1) – (n – 1)2
= 6n – 6 – (n2 – 2n + 1)
= 6n – 6 – n2 + 2n –1
= 8n – n2 – 7
an = Sn – Sn – 1
= 6n – n2 – 8n + n2 + 7
= –2n + 7
a25 = –2(25) + 7
= –50 + 7
= –43.
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