Question:
Choose the correct answer in each of the following questions:
If the nth term of the AP is (2n + 1) then the sum of its first three terms is
(a) 6n + 3
(b) 15
(c) 12
(d) 21
Solution:
nth term of the AP, an = 2n + 1 (Given)
∴ First term, a1 = 2 × 1 + 1 = 2 + 1 = 3
Second term, a2 = 2 × 2 + 1 = 4 + 1 = 5
Third term, a3 = 2 × 3 + 1 = 6 + 1 = 7
∴ Sum of the first three terms = a1 + a2 + a3 = 3 + 5 + 7 = 15
Hence, the correct answer is option B.
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