Chapter 12 Sequences and Series - 12.2 Analyze Arithmetic Sequences and Series - 12.2 Exercises - Skill Practice - Page 806: 28

$a_n=\dfrac{-1}{2}n+6$

We know that the general formula of an arithmetic sequence is given by $\dfrac{1}{2}= a_1+(11-1)-\dfrac{1}{2}$ Here, we have $a_1=\dfrac{11}{2} ; d=-13$ $a_n=\dfrac{11}{2}+(n-1) \times (-\dfrac{1}{2})$ This gives: $a_n=\dfrac{12-n}{2}$ Hence, $a_n=\dfrac{-1}{2}n+6$