The difference between two consecutive terms of an AP is called as its common difference (d).
Consider the arithmetic progression 10, 20, 30.
We have d=20−10=30−20=10.
For the AP 1.5, 3, 4.5, we have
d=3−1.5=1.5.
Similarly, for the AP -2, 0, 2, we have
d=0−(−2)=2.
And, for the AP -1, -1, -1, the common difference is given by
d=−1−(−1)=0.