If the nth term of an A.P. is (2n + 1), then the sum of its first three terms is
A. 6n + 3
B. 15
C. 12
D. 21
Answers
Answered by
6
option {a} is correct
Answered by
19
Answer:
Given :
nth term = 2 n + 1
= > a₁ = 2 + 1
= 3
= > a₂ = 4 + 1
= 5
d = a₂ - a₁
= > 5 - 3
= 2
Sₙ = n / 2 [ 2 a + ( n - 1 ) d ]
S₃ = 3 / 2 [ 6 + 2 × 2 ]
S₃ = 3 / 2 [ 10 ]
S₃ = 3 × 5
S₃ = 15 .
Therefore , sum of first three numbers is 15 .
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