The History of Mathematics in School for Grades IXX is a continuation of the first two books for grades published in 1981 and 1982. The first part of the book contains material on the history of mathematics, corresponding to the program of IX and X classes, and the second part is intended for extracurricular classes with students or for independent reading by students who are fond of mathematics and its history. At the end of the book, historical problems are placed with instructions for their solution and answers. This book is distinguished from previous ones not only by its content, but also by a stricter presentation of the material, which is due to the age characteristics and development of high school students. In separate articles for extracurricular classes there is some theoretical information that is not available in the secondary school program, for example; continuous fractions, complex numbers, probability theory issues, etc. However, this material can be used for lessons in a mathematical school or in a technical school. The book will be useful to teachers, students of pedagogical schools, schoolchildren, as well as everyone interested in mathematics and its history.