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    Question

    If the sum of n numbers in the GP 3, 6, 12, ... is 765

    then n is?
    A 12 Correct Answer Incorrect Answer
    B 6 Correct Answer Incorrect Answer
    C 8 Correct Answer Incorrect Answer
    D 4 Correct Answer Incorrect Answer

    Solution

    Given series is 3, 6, 12, ... Here, a = 3, r = 2 Sum of n numbers = Sn= 765 As we know that, Sum of n terms of GP = Sn = a(rn – 1)/(r – 1); where r >1 Sn = 765 = 3 × (2n – 1) ⇒ 255 = (2n – 1) ⇒ 2n = 256 ⇒ 2n = 28 ⇒ n = 8

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