You should know how the high-order infinitesimal α comes from! Since its limit exists and is 0, there must be | α-0 | < C (I use c here to represent any number greater than 0), that is | α| < C, and because B is greater than 0, I take C=b/2, that is | α |.

I feel that it has been done in detail. You can ask me if you don't understand.