Make a subtraction table and find that if you look at it vertically, you can find that the reduction is unchanged and the gain number changes with the change of the minuend. Subtraction is a mathematical operation that represents the operation of removing an object from a collection. Subtraction is to remove or reduce physical quantities and abstract quantities with different objects (including negative numbers, fractions, irrational numbers, vectors, decimals, functions and matrices).

The definition of function is usually divided into traditional definition and modern definition. The two definitions of functions are essentially the same, but the starting point of describing concepts is different. The traditional definition is from the perspective of movement change, and the modern definition is from the perspective of set and mapping. The modern definition of a function is to give a number set A, assume that the element in it is X, apply the corresponding rule F to the element X in A, and record it as f(x) to get another number set B, assume that the element in B is Y, and the equivalent relationship between Y and X can be expressed as y=f(x). The concept of a function contains three elements: the definition field A, the value field C and the corresponding rule F, among which the core is the corresponding rule F, which is the essential feature of the function relationship.