E-books, calculus. In this paper, ex is an e-book paper and dy is calculus. When the change of x is small, the corresponding change of y is also small. Papers are often used to refer to articles that conduct research in various academic fields and describe academic research results, which are referred to as papers for short. It is not only a means to discuss problems in academic research, but also a tool to describe academic research results in academic exchanges. Including academic papers, graduation papers, dissertations, scientific papers, achievement papers and so on.