Definition

1. Noun:
   • Diagonal: In geometry, a straight line segment that connects two non-adjacent vertices (corners) of a polygon or a polyhedron. It is a fundamental concept in mathematics, particularly in the study of shapes.

Usage Examples

• Noun:
  • Hình vuông có hai đường chéo bằng nhau. (A square has two diagonals of equal length.)
  • Để tính chiều dài đường chéo của hình chữ nhật, bạn có thể sử dụng định lý Pythagoras. (To calculate the length of the diagonal of a rectangle, you can use the Pythagorean theorem.)
  • Anh ấy vẽ một đường chéo chia hình chữ nhật thành hai tam giác. (He drew a diagonal dividing the rectangle into two triangles.)

Advanced Usage

• "Đường chéo chính" (Main/Principal Diagonal): In a square matrix, this refers to the diagonal of elements from the top-left to the bottom-right corner.

  • Tổng các phần tử trên đường chéo chính của ma trận được gọi là vết của ma trận. (The sum of the elements on the main diagonal of a matrix is called the trace of the matrix.)

• "Đường chéo phụ" (Secondary/Anti-diagonal): In a square matrix, this refers to the diagonal of elements from the top-right to the bottom-left corner.

Variants and Related Words

• Chéo (Adjective/Adverb): Diagonal, diagonally. Can describe something positioned or moving along a diagonal line.

  • Cắt tấm vải theo đường chéo. (Cut the fabric diagonally.)

• Đường chéo góc: A less common variant emphasizing the angular, corner-to-corner nature.

Synonyms

• Đường xiên: An oblique or slanting line. While similar, "đường xiên" is a more general term for any slanted line, not necessarily connecting non-adjacent vertices of a polygon.
• Đường chéo cánh (informal/slang): Used in some contexts, like sports ("chạy đường chéo cánh"), meaning to run diagonally across a field.

Related Phrases

• Cắt chéo: To cut diagonally.

  • Cắt bánh mì chéo để có miếng to hơn. (Cut the bread diagonally to get larger slices.)

• Nhìn chéo: To look sideways or askance.

  • Cô ấy nhìn chéo sang tôi với vẻ nghi ngờ. (She looked askance at me with suspicion.) (Note: This phrase uses "chéo" as an adverb, not directly related to the geometric "đường chéo").

Related Idioms

• : To speak indirectly, to beat around the bush. This idiom uses "chéo" metaphorically to imply non-straightforward, oblique speech.
  • Đừng nói chéo nói xiên nữa, hãy đi thẳng vào vấn đề. (Stop beating around the bush, get straight to the point.)

1. (toán) Diagonal