The steps to prove whether a given number sequence is an arithmetic progression

The steps to prove whether a given number sequence is an arithmetic progression.

Step 1: List down any three consecutive terms. [Example: T 1 , T 2 , T 3 .]
Step 2: Calculate the values of T 3 T 2  and T 2 T 1 .
Step 3: If T 3 T 2 = T 2 T 1 =d , then the number sequence is an arithmetic progression.

[Try Question 8 and 9 in SPM Practice 1 (Arithmetic Progression)]

Example :

Prove whether the following number sequence is an arithmetic progression
(a) 7, 10, 13,....
(b) -20, -15, -9 ,......