Problem 72 - College Algebra

Question

Evaluate: \(\left|\begin{array}{lllll}2 & 0 & 0 & 0 & 0 \\ 0 & 0 & 0 & 0 \\ 0 & 0 & 2 & 0 \\ 0 & 0 & 0 & 1 \\ 0 & 0 & 0 & 0\end{array}\right|\)

Step-by-Step Solution

Answer

The determinant of the given matrix is 0.

Step 1: Identify the Diagonal Elements

In given 4 by 4 matrix, the diagonal elements are 2, 0, 2 and 1. These are the elements from the top left to bottom right (main diagonal).

Step 2: Calculate the product of Diagonal Elements

The determinant of a diagonal matrix is the product of its diagonal elements. So just multiply these values: 2*0*2*1 = 0 as multiplication by zero equals zero.

Step 3: State the Determinant

The determinant of the given matrix is 0.

Key Concepts

Diagonal Matrix

A diagonal matrix is a special type of square matrix where all off-diagonal elements, those not on the main diagonal running from the top-left to the bottom-right, are zero. In other words, in a diagonal matrix, any element Aij where i ≠ j is zero. This unique structure makes computations involving diagonal matrices much simpler. For example, multiplying a diagonal matrix by a vector can be done quickly by just scaling each element of the vector by the corresponding diagonal element of the matrix.

Due to their simplicity, diagonal matrices are used in various mathematical applications, including solving systems of linear equations and diagonalization of matrices. Diagonalization, in particular, is the process of finding a diagonal matrix that is similar to a given square matrix, which can then be used to easily raise the original matrix to a power or to find its exponential.

Product of Diagonal Elements

The product of the diagonal elements of a matrix, often referred to as the 'trace' when added, is significant because it leads directly to the value of the determinant for diagonal matrices. Since a diagonal matrix D has non-zero elements only on its main diagonal, the determinant of D is simply the product of these elements. This property significantly simplifies the computation of determinants for diagonal matrices, as one does not need to perform the usually more complex Laplace expansion or row reduction.

To calculate the product, you would multiply each element found on the main diagonal with one another. Mathematically, if you have a diagonal matrix D with diagonal elements d1, d2, ..., dn, the product of the diagonal elements would be d1 x d2 x ... x dn. If even one of these diagonal elements is zero, the product, and hence the determinant, will be zero as well.

Properties of Determinants

Determinants offer a wealth of information about a matrix, mostly related to the matrix's invertibility and linear independence of its columns or rows. There are several key properties that determinants have which come in handy for computations and understanding the nature of a matrix:

• A determinant of a square matrix changes sign when two rows (or two columns) are swapped.
• If two rows (or two columns) of a matrix are identical, its determinant is zero.
• Adding a multiple of one row to another row does not change the determinant.
• The determinant of a matrix is the product of its eigenvalues.
• The determinant of the identity matrix is 1, and the determinant of a zero matrix is 0.
• If a matrix has a row or a column of zeros, its determinant is zero.
• The determinant of a matrix product equals the product of their respective determinants (det(AB) = det(A)det(B)).

These properties are crucial when solving linear algebra problems. They can make otherwise difficult problems more manageable and lead to quicker, more intuitive solutions. For instance, knowing that a matrix with a zero row has a determinant of zero allows us to immediately conclude the determinant without further calculations.