13-31 uses a binomial expansion about the bottom of a potential well to find that a vibrating atomic bond can be well modeled by simple harmonic motion (i.e., a spring). The mathematics is well worth your while. This makes a very real connection with all your work in chemistry. Though you might be tempted to blow it off as too mathematical … I think the stuff in section 13-5 of your text is very important for you. So, dig into it!