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Questions
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
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.
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