alleryHow many terms of the AP 3, 6, 9, … add up to 84?
A5
B6
C7
D8
Answer & Solution
Correct answer: C. 7
We have Sₙ = n/2[6 + (n − 1)·3].
That is Sₙ = 3n(n + 1)/2.
That is Sₙ = 84 ⇒ n(n + 1).
That is Sₙ = 56 ⇒ n.
So Sₙ = 7.
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