If a sequence is:

a. -2,2,-2,2,-2,....

b. 4,2,(4/3),1,(4/5),...

How do I prove the common difference, d, using the Arithmetic Sequence for these 2 problems?

ie.

a3 = a1 (n - 1)d

So would problem a be:

-2 = -2(3-1)d ?

Im getting 1/2 for d but its not the right answer

Thanks