Question

What is the common difference between successive terms in the sequence?

9, 2.5, –4, –10.5, –17, ...

–11.5
–6.5
6.5
11.5

Answer 1

Compute successive differences: 2.5 − 9 = −6.5, −4 − 2.5 = −6.5, etc.
Common difference = −6.5.