In mathematics, we should study all kinds of numbers and shapes. It is not inherent in people's minds, but what people get from social practice. Since human ancestors began to make tools, they have left the animal kingdom and have a certain understanding of the strange "shapes".

For example, the ancients observed that an angle was formed between the legs or between the upper and lower arms, and this image was repeated countless times in their minds, which may lead to the concept of ignorance of the angle.

With the continuous progress of society, people finally abstract its essential concept from images from various angles: a graph composed of two rays starting from a point is called an angle. The "angle" is represented by the symbol "∞" and pronounced as "angle". Angle is one of the simplest figures in geometry. Combining "∞" with several letters can visually represent an angle.