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The 4th term of an AP is 10 and the 9th term is 25.
Find the sum of its first 20 terms.

Class 10Sum of n Terms of an APAlgebra — Sequences

A590
B560
C610
D550

Step-by-Step Solution

a + 3 d = 10, a + 8 d = 25 \Rightarrow 5 d = 15 \Rightarrow d = 3, a = 1. S_{20} = \frac{20}{2} [2 (1) + 19 (3)] = 10 [2 + 57] = 10 (59) = 590

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Answer: A) 590

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