It is well known that the problems about the elastic collision of two bodies in 2-D space can not be exactly solved generally. It is because there are six unknown hut four equations. In order to solve this problem, we can appoint two variables as the parameters. For example, the angle between the incidence direction and scattering direction of the injected particle could be a good parameter. For more realistic we can suppose the two bodies are two discs. It is a complex but not profound problem. Most textbooks don't describe it completely owing to it being long-winded. The purpose of the paper is to develop the analysis of the textbook and plot some figures which can make readers more clear about scattering problems. It will attract readers' attention if they like billiard sport.