SOLUTION
The given sequence is:
[tex]7,21,63,189,\cdots[/tex]
The first term of the sequence is 7.
Notice that the sequence has a common ratio of:
[tex]r=\frac{21}{7}=\frac{63}{21}=\cdots=3[/tex]
The recursive formula of a geometric series is:
[tex]a_n=a_{n-1}(r)[/tex]
Substituting r=3 gives:
[tex]a_n=3a_{n-1}[/tex]
