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