How do I write equations for geometric sequences?

Question

example question:
1, 3/2, 9/4, 27/8

an= a(n-1) * r^n where r is the value of dividing one term by the term before.
For instance, in the above, ao= 1,

r= a3/a2= (27/8)/(9/4)= 3/2

a1= 1*3/2

a3= 1*(3/2)^3= 27/8

Thank you, Mr. Pursley