  This book is devoted to the basic concepts of angles and triangles in Euclidean plane geometry.n These concepts also form the basis of Euclidean geometry. However, these concepts are open to the Euclidean world and they can be used to describe geometry in other spaces. It will probably surprise many readers that, in addition to basic concepts from geometry, consideration of this topic requires the use of other axioms, such as the axiom of parallel lines, which describes the boundaries of triangles. Solving problems about geometry and geometry using these axioms will require some mathematical calculations that are rarely done in everyday life. We will consider this problem using the example of a right triangle. Although Euclidean space, like the space of higher spaces, contains many infinite planes and infinitely many lines, there are very few basic concepts of geometry in these spaces. In addition, these spaces do not have canonical, direct spaces that map them to other spaces. In the space of higher spaces, such as three-dimensional space, it is more relevant to use axium definitions for axion theorems: the parallel theorem and the projection and normalization theorems. By discussing these theorems, we will cover the basic concepts of geometry and find the basic methods for solving geometric problems. Then we will continue the discussion, consider more advanced topics such as area and volume problems using the Euclidean coordinate system. The author hopes that the information contained in the book will be equally useful and interesting for both schoolchildren and students.
The first edition of this book appeared in 1966, and there have been three subsequent editions.

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