Precalculus: Concepts Through Functions, A Unit Circle Approach to Trigonometry (3rd Edition)

$a_{80} = 157$
The $n^{th}$ term of an arithmetic sequence is given by the formula: $a_n = a_1 + (n-1)d (1)$ where $a_1 = \ First \ Term; \\ d = \ Common \ Difference$ We have: $a_1=-1 \\ d=a_2-a_1=3-1=2$ We will substitute the above data into formula (1) to obtain: $a_n=-1+(2) (n-1)$ In order to compute the $80th$ term, we need to plug in $80$ for $n$ in the above form to obtain: $a_{80} = -1+(2) (80-1) =-1+(79)(2)$ So, $a_{80} = 157$