Chapter 10 - Section 10.2 - Arithmetic Sequences - Exercise Set - Page 1059: 23

General equation is: $a_n=1+4(n-1)$ ; $77$

Formula to calculate the nth term of an arithmetic sequence is: $a_n=a_1+d(n-1)$ Here, $a_1: $ First term and $d=$ common difference Since, we have $a_1=1$ Then, our General equation is: $a_n=1+4(n-1)$ $a_{20}=1+4(20-1)=1+76=77$