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NCERT Class XI Mathematics - Sequences and Series - Solutions
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Question : 20 of 106
Marks:
+1,
-0
If the sum of a certain number of terms of the A.P. 25, 22, 19, .... is 116. Find the last term.
Solution:
Let a be the first term and d be the common difference. We have a = 25, d = 22 – 25 = –3, = 116 Since = [2a + (n - 1) d] ∴ 116 = [50 + (n - 1) (- 3)] ⇒ 232 = 50n – + 3n ⇒ – 53n + 232 = 0 ⇒ – 24n – 29n + 232 = 0 ⇒ (3n – 29) (n – 8) = 0 ⇒ n = , 8 Since n ≠as n can not be in the fraction form ∴ n = 8 Hence, the last term = a + 7d = 25 + 7 (– 3) = 25 – 21 = 4.
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