Let a_1, a_2, a_3, a_n, be in A.P. If a_3+a_7+a_ 11 +a_ 15 = 72, then the sum of its first 17 terms is equal to :

JEE Main 2016 Mathematics question

Let , , be in A.P. If , then the sum of its first 17 terms is equal to :

Answer: (1)

In an arithmetic progression, terms equidistant from a given term average to that term. Here the four indices pair up symmetrically about 9: and , so

+ = and + = .

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Question ID VK-115647