1.2, 3, 7.5, 18.75, …  Which formula can be used to describe the sequence?

NetherCraft 0

3 Answers

  • 1.2 ( 5/2)^n , n = 0,1,2,

  • 1.2, 3, 7.5. 18.75, …

    an = 3 (2/5)^(2 – n) 

  • Check for an arithmetic sequence.

    The difference between adjacent terms is not constant, so you know it is not an arithmetic sequence.

    Check for a geometric sequence:

    a₂/a₁ = 3/1.2 = 2.5 

    a₂×2.5 = 7.5 = a₃

    a₃×2.5 = 18.75 = a₄

    So it is a geometric sequence with ratio 2.5

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