Question

A phone plan gives a student 1 GB of data the first month, and the amount of data triples every month due to promotions.



Which recursive formula correctly represents this geometric sequence?



A.
a
1
=
3,

a
n
=
1
⋅
a
n
−
1

B.
a
1
=
3,

a
n
=
1
⋅
a
n
+
1

C.
a
1
=
1,

a
n
=
3
⋅
a
n
−
1

D.
a
1
=
1,
a
n
=
3
⋅
a
n
+
1

Answer 1

C.

Explanation: a1 = 1 (first month 1 GB). Each month triples, so for n ≥ 2, an = 3·a_{n−1}. (Thus a2 = 3, a3 = 9, etc.)