Question

What is the common difference in the sequence 9, 4, -1, -6?

A. 5

B. -5

C. 9

D. -11

(Am I dividing or subtracting?)

Answer 1

Use first differences. You are subtracting. Difference = Subtracting.

9 - x = 4
4 - x = -1
-1 - x = -6

Find for x.

9 - x = 4
   - x = 5
     x = -5

Therefore the answer is B.

Proof:

9 - 5 = 4
4 - 5 = -1
-1 - 5 = -6