The main diagonal is defined as: in an n-order square matrix (or n-order determinant), the positions of n elements on the diagonal from the upper left corner to the lower right corner are called n The main diagonal of an order square matrix (or determinant).

The determinant of order n represents n, and the algebraic sum of the terms accounts for half of the positive and negative terms. Each term is the product of n elements in different rows and different columns. The diagonal is called the main diagonal of the determinant.

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The meaning of linear algebra continues to expand with the development of mathematics. The theories and methods of linear algebra have penetrated into many branches of mathematics, and are also indispensable algebraic basic knowledge in theoretical physics and theoretical chemistry.

"Replacing music with straight lines" is a natural thought when people deal with many mathematical problems. The processing of many practical problems often boils down to linear problems, which are easier to deal with. Therefore, linear algebra is widely used in many fields of engineering technology and national economy, and is a basic and important subject.