Chapter 11 - 11.2 - Arithmetic Sequences and Partial Sums - 11.2 Exercises - Page 786: 26

$a_n=15-5n$

The nth term of an arithmetic sequence can be obtained by the following formula: $a_n=a_1+(n-1)d$, where $a_1$ is the first term and $d$ is the common difference. Hence here: $a_1=10,d=a_2-a_1=5-10=-5$ Thus $a_n=a_1+(n-1)d=10+(n-1)-5=10-5n+5=15-5n$