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:
an=a1+(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. | 
