Equations with unknowns are equations, and mathematics was first developed in counting. The combination of addition, subtraction, multiplication, division and idempotency between numbers and unknowns forms algebraic equations: one-dimensional linear equations, one-dimensional quadratic equations, two-dimensional linear equations and so on. However, with the emergence of the concept of function and the introduction of differential and integral operation based on function, the category of equation is wider, and the unknowns can be mathematical objects such as functions and vectors, and the operation is no longer limited to addition, subtraction, multiplication and division.

Equation occupies an important position in mathematics and seems to be an eternal topic in mathematics. The appearance of equations not only greatly expanded the application scope of mathematics, but also solved many problems that could not be solved by arithmetic problem-solving methods, which had a great influence on the progress of mathematics later. In particular, many important discoveries in mathematics are closely related to it. For example:

The solution of quadratic equation leads to the discovery of imaginary number;

The solution of quintic and above equations leads to the birth of group theory;

The study of linear equations leads to the establishment of linear algebra, and the study of polynomials leads to the emergence of polynomial algebra.

Applying equations to solve geometric problems leads to the formation of analytic geometry and so on.

The equations in middle schools are basically of this kind, and the unknowns in the equations can appear in the independent variables of fractions, algebras, roots, trigonometric functions and exponential functions. For example, the following form (x, y are unknowns):

We have the problem of solving equations in middle school. Generally speaking, we can convert equations into integral equations. Generally, it is transformed into a quadratic equation with one variable, or the solution of a system of linear equations with multiple variables.

Because mathematics has changed from constant mathematics to variable mathematics, the contents of equations are enriched, because mathematics has introduced more concepts and more operations, thus forming more equations. The development of other natural sciences, especially physics, also directly puts forward the demand of solving equations and provides a large number of research topics.