Multiple Choice Question (MCQ)

How many terms of the series 1 + 3+ 9+…. sum to 121?

  1. ×

    3

  2. ×

    4

  3. 5

  4. ×

    6

Collected in the board: Geometric sequence

Steven Zheng posted 3 months ago


Answer

  1. The given series is a geometric sequence with

    a_1 =1 , and r=3


    Sum of n terms of a geometric sequence is

    S_n = a_1 * (r^n – 1)/ (r – 1)

    S_n =a_1 \dfrac{r^n – 1}{r – 1}


    Therefore,


    121 = \dfrac{3^n-1}{3-1}


    \therefore n =5

Steven Zheng posted 3 months ago

Scroll to Top