Geometric Sequence:
Always MULTIPLY by the same amount to get from one term to the next
Amount you multiply by = "Common Ratio" = r
If you're ever wondering what the common ration is, pick a random term and divide by the term before it.
Arithmetic Sequence
Always ADD the same amount to get from one term to the next
Amount you add = "Common Difference" = d
If you're ever wondering what the common difference is, pick a random term and subtract the term before it.
A) 3, 8, 13, 18, … 
B) 4, 8, 16, 32, … 
C) 27, −18, 12, −8, … 
D) 1, −3, 5, −7, …