Chapter 12 Sequences and Series - 12.2 Analyze Arithmetic Sequences and Series - 12.2 Exercises - Skill Practice - Page 806: 15

$a_n=-5+2n$ ; $a_{20}=35$

We know that $a_n= a_1+(n-1) d$ Here, we have $d=-1-(-3)=2$ $a_n= (-3)+(n-1) \times 2$ This gives: $a_n=-5+2n$ When $n=20$, we have $a_{20}= -5+2 \times 20=35$ Hence, $a_n=-5+2n$ ; $a_{20}=35$