Step-by-step explanation:
arithmetic sequence means every new term is the previous term plus a certain constant.
as we can see, this constant is +4 in our case.
15 + 4 = 19
19 + 4 = 23
23 + 4 = 27
...
so, we have
a1 = 15
a2 = a1 + 4 = 15 + 4 = 19
a3 = a2 + 4 = a1 + 4 + 4 = a1 + 2×4 = 15 + 8 = 23
...
an = an-1 + 4 = a1 + (n-1)×4 = 15 + (n-1)×4
