# How do you write a rule for the nth term of the geometric term given the two terms #a_1=1, a_3=9#?

##### 1 Answer

May 21, 2017

There are two possible common ratios and corresponding sequences, given by the formula:

#a_n = 3^(n-1)" "# or#" "a_n = (-3)^(n-1)#

#### Explanation:

The general formula for a term of a geometric sequence is:

#a_n = ar^(n-1)#

where

In our example, we find:

#9 = a_3/a_1 = (ar^2)/a = r^2#

Hence

The rule for the n

#a_n = 3^(n-1)#

or:

#a_n = (-3)^(n-1)#