The book is a continuation of the books "Behind the pages of a mathematics textbook. Arithmetic. Algebra" and "Behind the pages of a mathematics textbook. Geometry. Ancient and entertaining tasks" and is addressed to high school students who want to expand and deepen their knowledge of all branches of mathematics. The presentation of new mathematical concepts is based on the school course and is accompanied by interesting historical facts. The book immerses students in the world of modern mathematics, talks about the role of mathematical scientists in the development of world science. Theoretical information is supplemented by a variety of tasks.