Answer
General formula: $a_n=1+(n-1)(4)$
$a_{20} = 77$
Work Step by Step
The formula for the general term (the nth term) of an arithmetic sequence is:
$a_n=a_1 + (n-1)d$
where
d = common difference
$a_1$ = first term
$a_n$ = nth term
n = term number
To find the formula for the general term, perform the following steps:
(1) Find the values of $a_1$ and $d$
The given sequence has:
$a_1 =1$
$d= 5 -1 = 4$
(2) Substitute the values of $a_1$ and $d$ in the general formula given above to find:
$a_n= 1 + (n-1)(4)$
Therefore, the 20th term of the sequence is:
$a_{20} = 1 + (20-1)(4)
\\a_{20} = 1+19(4)
\\a_{20} = 1 + 76
\\a_{20} = 77$
