Chapter 12 - Sequences; Induction; the Binomial Theorem - 12.2 Arithmetic Sequences - 12.2 Assess Your Understanding - Page 814: 29

$\frac{83}{2}.$

We know that $a_n=a_1+(n-1)d.$ where $d$=common difference and $a_1$= the first term Here $a_1=2$ and $d=a_2-a_1=\frac{5}{2}-2=\frac{1}{2}$ Hence, here we have $a_n=2+(n-1)\cdot\frac{1}{2}.$ Therefore $a_{80}=2+(80-1)\cdot\frac{1}{2}\\=\frac{83}{2}.$