Answer:
-1, -6, 13, 20 is an arithmetic sequence
d = 7
5, 12, 19, 26 is arithmetic sequence
d = 7
-3 - 9, -27, -81 is not an arithmetic sequence
Explanation:
Common difference is the difference between successive terms and must be the same for all terms
-1, -6, 13, 20 is an arithmetic sequence with common difference:
6 - (-1) = 13 - 6 = 20 - 13 = 7
5, 12, 19, 26 is arithmetic sequence with common difference
12- 5 = 19 - 12 = 26 - 19 = 7
-3 - 9, -27, -81 is not an arithmetic sequence
The difference between successive terms is not the same
-9 - (-3) = -9 + 3= -6
but
-27 - (-9) = -27 + 9 = -18
In fact it is a geometric sequence where each term = x previous term
-9 = 3 x -3
-27 = 3 x -9
etc