the formula is an =a1+d(n-1)
where a1 is the first terms
d is the common difference
a1= 19
d 1= 9-19 = -10
check: -1-9 = -10 the sequence goes down by 10
n is the term number
an = 19 - 10(n-1)
we can distribute
an = 19 -10n +10
an = 29-10n
Check our work:
a4 = 29 - 10*4
= 29 - 40
= -11
the 4th term is -11 which 19,9,-1,-11,... shows, so we are good
Answer: an = 19 - 10(n-1) or an = 29-10n
