So this leads directly to the following scene. One day, a classmate was studying with me in the library. I kept hearing voices like DX and DY coming from the side, provided that I was studying integral. So I suddenly understood that everyone was talking about dx dy. Please pay attention to the pronunciation of this d and the discriminant in the quadratic function of one variable. So I think it is necessary to have a comprehensive and systematic understanding of the pronunciation of various Greek letters. In fact, this process is not very easy. Don't talk to me about Baidu Encyclopedia. Baidu Encyclopedia is a relatively stupid thing. For example, in its entry on high numbers, the concepts of infinite series and sequence are confused (I have corrected this). Students who have studied advanced mathematics know that these two concepts are not the same thing at all. For another example, Baidu Encyclopedia, which you think you know everything, has no correct content in the entry of electronic system design. In fact, it is two disciplines under the first-level discipline of computer science and technology. After many twists and turns, I found a more reliable pronunciation table. To my shame, the beta I have studied for many years is actually Bita. . . . (Published by Department of Applied Mathematics, School of Science, South China University of Technology, Ministry of Education, National Engineering Mathematics Teaching Base, Higher Education Press, Higher Education Electronic Audio-visual Press) Things should have come to an end, but the question is, where is the partial derivative symbol? My classmates may despise me and say that he is biased. Looking back on the classroom at that time, Professor Zhou, who has always been regarded as a great god by me, only read it slightly in the face of more than 20 pairs of eager eyes. The big concept is to exclude the west. Some students who have been exposed to foreign calculus textbooks may laugh when they see this, so don't worry. On the contrary, they suggest that we study a little. Don't! Calculus is not summed up by China people, and obviously it can't be a reading deviation. Then, I began to reverse the sign of deviation value and partial derivative. The following paragraph is actually very boring, and there are many gnashing gods that some students hate. Start from the beginning. Calculus equation was born in18th century. Engels once said that the invention of calculus is the highest victory of human spirit. 1687, Newton published his calculus theory for the first time in the book Mathematical Principles of Natural Philosophy. Almost at the same time, Leibniz also published a paper on calculus, but the foundation of calculus founded by Newton and Leibniz is unstable and its application scope is limited. Hu Zuoxuan, a historian of mathematics, believes that Newton made a breakthrough, but the breakthrough did not necessarily form a discipline, and there are still many problems left over. For example, Newton's definition of infinitesimal is not strict, sometimes it is equal to zero, and sometimes it participates in operation, so it is called the ghost of vanishing quantity. At that time, even the pastor of the church seized this point and attacked Newton. In addition, due to the limitations of functions at that time, Newton and Leibniz only involved a small number of functions and their calculus solutions. The really awesome person is actually Euler! I can't help but want to make up hundreds, thinking that probably no one will read it, that's all. . . . Here comes the theme. We have entered a one-sided era. At that time, Euler put forward the second-order equation of string vibration for the first time in his works. Not long after, the French mathematician D'Alembert also proposed a special partial differential equation in his book Dynamics. But none of these works attracted much attention at that time. 1746, D'Alembert proposed in his paper "Research on Curves Formed by String Vibration" that it is necessary to prove that infinitely many curves different from sine curves are vibration modes. In this way, the study of string vibration created the discipline of partial differential equations. Please don't tell me calculus is isaac newton or Leigh.