If a₁, a₂, a₃, ……. are in A.P. such that a₁ + a₇ + a₁₆ = 40, then the sum of the first 15 terms of this A.P. is:
(A) 200 (B) 280
(C) 150 (D) 120
Answers
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Step-by-step explanation:
a+a+6d+a+15d = 40
3a + 21d = 40
a + 7d = 40/3
S(15) = 15/2 {2a + 14d}
= 15/2 {2(a+7d)}
= 15{40/3}
= 5*40 = 200
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