Step-by-step explanation:
an arithmetic sequence means that any term is the result of the previous term + something.
a1 = 17
then we see that every term is the previous term -4.
a2 = a1 - 4
a3 = a2 - 4 = a1 - 4 - 4 = a1 - 8
so, we see
an = an-1 - 4
an = a1 - 4×(n-1)
