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