Answer
$a_{80} = 157$
Work Step by Step
RECALL:
(1) The $n^{th}$ term of an arithmetic sequence can be found using the formula:
$a_n = a_1 + (n-1)d$
where $a_1$ = first term and $d$ = common difference
(2) The common difference $d$ can be found by subtracting any term to the next term of the sequence:
$d=a_n - a_{n-1}$
The given sequence has:
$a_1=-1$;
$d=3-1=2$
Substitute these values into the formula for the $n^{th}$ term, we obtain:
$a_n=a_1 + (n-1)d
\\a_n=-1+(n-1)2$
To find the 80th term, substitute $80$ for $n$ to obtain:
$a_{80} = -1+(80-1)(2)
\\a_{80}=-1+79(2)
\\a_{80}=-1+158
\\a_{80} = 157$
