Chapter 12 Sequences and Series - 12.2 Analyze Arithmetic Sequences and Series - 12.2 Exercises - Skill Practice - Page 807: 31

$a_n=9+5n$

We know that the general formula of an arithmetic sequence is given by $a_n= a_1+(n-1) d$ ...(1) Here, we have $a_{6}=39$ $a_{14}=a_6+8d=79$ This gives: $d=5$ $a_1=a_6-5d=39-5 \times 5=14$ Now, $a_n=14+(n-1)5=9+5n$ Hence, our answer is $a_n=9+5n$