Answer
Formula for $a_{n}$ term of the given sequence = $a_{1}$ + (n - 1) d.
Where
$a_{1}$ = first term
d = Common difference
n = $n^{th}$ number of term
$20^{th}$ term of sequence = - 89
Work Step by Step
Given arithmetic sequence = 6, 1, -4, -9- - - - -
Common difference between consecutive term d = (-9) - (-4) = (-4)- 1 = 1 - 6 = (-5).
Formula for $a_{n}$ term of the given sequence = $a_{1}$ + (n - 1) d.
Where
$a_{1}$ = first term
d = Common difference
n = $n^{th}$ number of term
$20^{th}$ term of sequence = 6 + (20 - 1) $\times$ (-5)
= 6 + 19$\times$(-5)
= 6 - 95
= - 89
