Chapter 12 - Sequences; Induction; the Binomial Theorem - 12.2 Arithmetic Sequences - 12.2 Assess Your Understanding - Page 814: 21

$a_n=0.5n-0.5.$ $a_{51}=25.$

We know that the $n^{th}$ term $a_n$ of an arithmetic sequence is given by the formula $a_n=a_1+(n-1)d$ where $d$=common difference and $a_1$= the first term Hence, here we have $a_n=0+(n-1)\cdot0.5\\a_n=0.5n-0.5.$ Therefore $a_{51}=0.5\cdot51-0.5\\a_{51}=25.5-0.5\\a_{51}=25.$