Chapter 11 - 11.2 - Arithmetic Sequences and Partial Sums - 11.2 Exercises - Page 786: 25

$a_n=6.5-2.5n$

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: $a_1=4,d=a_2-a_1=1.5-4=-2.5$ Thus $a_n=a_1+(n-1)d=4+(n-1)-2.5=4-2.5n+2.5=6.5-2.5n$