One day, Descartes (1596- 1650, French philosopher, mathematician and physicist) was ill in bed, but his thoughts never stopped. He kept thinking about a problem: geometry is intuitive, while algebraic equations are abstract. Can we use geometry to express the equation? Here, the key is how to link the points that make up the geometric figure with each group of "numbers" that satisfy the equation. He tried to figure it out. In what way can "point" and "number" be linked? Suddenly, he saw a spider in the corner of the roof and pulled down the silk. After a while, the spider climbed up along the silk and drew left and right. The spider's "performance" made Descartes' thinking suddenly enlightened. He thought, you can think of a spider as a point. It can move up and down, left and right in the room. Can you determine every position of the spider with a set of numbers? He thought that two adjacent walls in the room gave three lines to the ground. If the angle on the ground is taken as the starting point and the three intersecting lines are taken as the three axes, isn't the position of any point in space expressed by the sequential three numbers found on these three axes? Conversely, a set of three series can be given arbitrarily, such as 3, 2, 1, and they can also be represented by a point p in space (as shown in figure 1). Similarly, a set of numbers (a, b) can represent a point on a plane, and a point on a plane can also be represented by a set of two consecutive numbers (as shown in Figure 2). So under the inspiration of spiders, Descartes created a rectangular coordinate system.