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How many terms of the AP 3, 5, 7, 9, ... add up to give a sum of 120?

Class 10Sum of n Terms of an APAlgebra — Sequences

A9
B11
C10
D20

Step-by-Step Solution

a = 3, d = 2, S = 120.120 = \frac{n}{2} [6 + 2 (n - 1)] \Rightarrow 240 = n (2 n + 4) \Rightarrow 2 n^{2} + 4 n - 240 = 0 \Rightarrow n^{2} + 2 n - 120 = 0 \Rightarrow (n + 12) (n - 10) = 0 \Rightarrow n = 10

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Answer: C) 10

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