If the nth term of an AP is (2n + 1) then find the sum of its first three terms - Mathematics

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If the nth term of an AP is (2n + 1) then find the sum of its first three terms

If the nth term of an A.P. is (2n + 1), then the sum of its first three terms is

Options

  • A. 6n + 3

  • B. 15

  • C. 12

  • D. 21

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Solution 1

∵ an = 2n + 1

a1 = 2(1) + 1 = 3

a2 = 2(2) + 1 = 5

a3 = 2(3) + 1 = 7

∴ a1 + a2 + a3 = 3 + 5 + 7 = 15

Solution 2

Given: The nth term of A.P. i.e., an = 2n + 1

To find: Sum of first three terms

On putting n = 1, 2 and 3, we obtain:

a1 = 2 × 1 + 1 = 2 + 1 = 3

a2 = 2 × 2 + 1 = 4 + 1 = 5

a3 = 2 × 3 + 1 = 6 + 1 = 7

∴ Sum of first three terms a1 + a2 + a3 = 3 + 5 + 7 = 15

Hence, the correct answer is B.

  Is there an error in this question or solution?
2011-2012 (March) All India Set 1

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