The concept of function was first put forward by German mathematician Leibniz in17th century. At first, Leibniz used the word "function" to represent power, such as y=kx+b, which are all called functions. Later, he used functions to represent the abscissa and ordinate of a point on a curve in a rectangular coordinate system. 17 18, Bernoulli, a student of Leibniz and a Swiss mathematician, defined a function as: "a quantity composed of a variable and an arbitrary constant." It means that any formula composed of variable X and constant is called a function of X, and Bernoulli emphasized that functions should be expressed by formulas. Later, mathematicians felt that the concept of function should not be limited to formulas. As long as some variables change, other variables can also change accordingly.