B here, like A, stands for a number.

Can represent any real number,

For example, Sn=3n? +2n，Sn=2n？ +(- 1/2)n and so on.

Arithmetic progression's tolerance is d, and the first term is a 1.

Then d = 2a and a+b = a 1.

∴ a 1=a 1-(d/2)