In simpler terms, the arithmetic means are the terms between any nonconsecutive terms in an arithmetic sequence.
You can determine arithmetic means between any number of terms in a sequence:

IE find four arithmetic means between -11 and 19 ...

The formula for any term n in an arithmetic sequence:

a

_{n}=a

_{1}+(n-1)d

19=-11+(6-1)d

(19 is the first

term, then there's four terms, plus the last term is six)

19=-11+5d

8=5d

d=5/8

start with 19 and then add 5/8 to 19, 5/8 to that sum (19 5/8), etc.

Not the most elegant math, but it works.