What are the values of a1 and r of the geometric series?

1 + 3 + 9 + 27 + 81

A. a1=1 and r=1//3

B. a1=1/3 and r=1

C. a1=1 and r=3

D. a1=3 and r=1

A1 or just a as it is in the equations is just the initial term of the sequence. In this case a1=1

r is the common ratio which is that constant ratio found by dividing any term by the term preceding it ...

In this case r=3/1=9/3=27/9=etc=3

So a1=1 and r=3, C. is your answer.