You may be trying hard to make something difficult out of something easy here.
You have said that a(n) = 8n . (I can't write a subscript 'n'on this site either,
but 'a(n)' will serve the same purpose.)
So . . . (get ready) . . .
a(n) = (8) times (n)
a(1) = 8
a(2) = 16
a(3) = 24
a(4) = 32
a(5) = 40
a(6) = 48
.
.
a(100) = 800
a(101) = 808
.
.
etc.