Generally, there are two important rules in the solution of triangle, namely the sine rule and the cosine rule. This leads most people to think that we need to learn the sine and cosine rules in order to solve all triangle problems, which is not true.In fact, all triangle problems can be solved using the cosine rules . But if all triangle problems can be solved with the cosine rules, why we need to learn Sine rules? This book will tell you the reasons why we need to learn the sine rule.