Chapter 12 Sequences and Series - 12.2 Analyze Arithmetic Sequences and Series - 12.2 Exercises - Skill Practice - Page 806: 14

$a_n=-5+13n$ ; $a_{20}=255$

We know that $a_n= a_1+(n-1) d$ Here, we have $d=21-8=13$ $a_n= 8+(n-1) \times 13$ This gives: $a_n=-5+13n$ When $n=20$, then we have $a_{20}= -5+13 \times 20=255$ Hence, $a_n=-5+13n$ ; $a_{20}=255$