Before we get started with the basics of secondary school mathematics, let's start with the greatest invention of mankind.
We modern people know what it is as soon as we see "=", and perhaps not all of us can clearly express its meaning and the most fundamental meaning, but we must know where it can be used and what it means.
In fact, "=" is essentially a symbol that expresses relations, and its mathematical significance lies in telling you that the left and right sides of the symbol are equal.Sounds simple, right?
In the 16th century and before, people did not begin to use the symbol system that is now common, and words have always been used to represent the equal relationship between two quantities, so that some mathematical books and articles are full of words such as aequ or aequaliter to indicate the equal relationship between two quantities.
In 1557, the English mathematics educator Robert Recorde  (c. 1510-1558), in his paper The Wheelstone of Witte, said: "In order to avoid boring repetition of the word isaequalleto (equal), I carefully compared many figures and marks, and felt that there was no more similar meaning in the world than two parallel and equal-length lines."Thus, Reicold creatively uses two parallel and equal line segments "=" to mean "equal", and "=" is called sign of equality.
Replacing the word "=" with the word equal is an important advance in mathematics.
We are talking about "=" here, not just about the life, severity, and effect of the symbol.There's a very, very important point of view and meaning involved, and that's all sciences, and the core idea of conservation.
In the simplest terms, it is on both sides of the "=", no matter how you change, no matter how you change, you must ultimately ensure that the two sides of "=" are always equal.
The first level of conservation, the understanding of the world, grasp the nature of things, understand that everything is so, is determined by its inherent invariable characteristics, this is the conservation of the first level.
The second level of intermediate conservation, looking for relationships, understanding the relationship between things, everything is not an isolated existence, but there are a variety of relationships in which to calmly grasp the relationship between things.
 雷科德（Robert Recorde，约1510～1558），英国数学教育家雷科德，除了数学、物理学、医学和矿物学，他还精通修辞、哲学、风 雅文学、历史、宇宙演化、天文学、星占术和音乐，是一位百科全书式的学者。出版了四部教材：《艺术基础》（1543）、《知识之途》（1551）、《知识城堡》（1551）和《砺智石》（1557），分别为算术、几何、天文和代数教材（其中《砺智石》是英国历史上第一部代数学教材）。