Chapter 14 - Sequences, Series, and the Binomial Theorem - 14.1 Sequences and Series - 14.1 Exercise Set - Page 894: 24

$a_{1}=2(1)+1=3$ $a_{2}=2(2)+1=5$ $a_{3}=2(3)+1=7$ $a_{4}=2(4)+1=9$ $a_{10}=2(10)+1=21$ $a_{15}=2(15)+1=31$

If we want to find a term, we have to substitute $n$ by its index: $a_{1}=2(1)+1=3$ $a_{2}=2(2)+1=5$ $a_{3}=2(3)+1=7$ $a_{4}=2(4)+1=9$ $a_{10}=2(10)+1=21$ $a_{15}=2(15)+1=31$