1. Because addition, subtraction, multiplication and division can be very complicated, many practical problems cannot be solved by elementary school mathematics algebra, and need to be applied to quadratic equations, irrational equations, fractional equations and so on. Junior high school students still need to learn deeper addition, subtraction, multiplication and division operations.

2. Letters represent numbers, and junior high school defaults to real numbers. It has auxiliary significance in concrete problems or simple analytical formulas or equality inequality problems, or expresses certain meaning to constrain the results and get a solution set (this is difficult to express, for example, the known number A is any real number, any non-zero number, etc. ), or simply test your thinking ability and discuss it in categories.