Chapter 12 Sequences and Series - 12.2 Analyze Arithmetic Sequences and Series - 12.2 Exercises - Skill Practice - Page 807: 33

$a_n=22-4n$

We know that the general formula of an arithmetic sequence is given by $a_n= a_1+(n-1) d$ ...(1) Here, we have $d=-4$; $a_8=-10; a_{20}=-58$ Now, we have $a_n=a_8+(n-8)d$ This gives: $a_n=-10+(n-8)(-4)=22-4n$ Hence, our answer is $a_n=22-4n$