Chapter 12 Sequences and Series - 12.2 Analyze Arithmetic Sequences and Series - 12.2 Exercises - Skill Practice - Page 806: 20

$a_n=-0.6+2.1 n$ ; $a_{20}=41.4$

We know that $a_n= a_1+(n-1) d$ Here, we have $d=3.6-1.5=2.1$ $a_n= 1.5+(n-1) \times (2.1)$ This gives: $a_n=-0.6+2.1 n$ $a_{20}=-0.6+2.1 \times 20=41.4$ Hence, $a_n=-0.6+2.1 n$ ; $a_{20}=41.4$