an aritmetic sequcne is represented as

where
an=nth term
a1=first term
d=common differnce, or how much each term increases by
n=which term
example
1,3,5, etc
first term is 1
common difference is 2 because it increases by 2 each time
so the formula would be

if we had
5,3,1
first term s 5
common difference is -2 since it goes up by -2 each time
so formula is

hope this helps