Chapter 5 - Number Theory and the Real Number System - 5.7 Arithmetic and Geometric Sequences - Exercise Set 5.7 - Page 329: 45

General Formula: $a_n=9+(n-1)(2)$ $a_{20}=47$

The formula for the general term (the nth term) of an arithmetic sequence is: $a_n=a_1+(n−1)d$ where d = common difference $a_1$ = first term $a_n$ = nth term n = term number The given arithmetic sequence has: $a_1=9$ and $d=2$ Substitute these into the given formula above: $a_n = 9 + (n-1)(2)$ Thus, $a_{20} = 9+(20-1)(2) \\a_{20}=9+19(2) \\a_{20}=9+38 \\a_{20}=47$