Chapter 1: Problem 38
Prove that, if \(A\) is a square matrix, then the matrix \(A+A^{T}\) is symmetric.
Short Answer
Expert verified
The matrix \(A + A^{T}\) is symmetric because \((A + A^{T})^{T} = A + A^{T}\).
Step by step solution
01
Understand Matrix Transposition
Matrix transposition involves converting a matrix such that its rows become columns. If we have a matrix \(A\), the transpose of \(A\) is represented as \(A^{T}\). For a symmetric matrix, the property \(A = A^{T}\) holds true.
02
Define the Expression to Prove
We need to show that the matrix \(A + A^{T}\) is symmetric. This requires proving that \((A + A^{T})^{T} = A + A^{T}\).
03
Apply the Transpose Property to the Expression
Use the property of transpose on the sum of two matrices: \((B + C)^{T} = B^{T} + C^{T}\). Apply this property to \((A + A^{T})^{T}\), which gives us \(A^{T} + (A^{T})^{T}\).
04
Simplify the Expression Using Properties of Transpose
Recognize that \((A^{T})^{T} = A\). Therefore, \(A^{T} + (A^{T})^{T}\) simplifies to \(A^{T} + A\).
05
Compare the Simplified Expression to the Original
Observe that \(A^{T} + A\) is the same as \(A + A^{T}\) since addition is commutative. This leads to \((A + A^{T})^{T} = A + A^{T}\), satisfying the condition for symmetry.
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Key Concepts
These are the key concepts you need to understand to accurately answer the question.
Matrix Transposition
Matrix transposition is a fundamental operation in linear algebra. It involves swapping the rows and columns of a matrix. For a matrix denoted as \( A \), its transpose is represented by \( A^{T} \).
- Each element \( a_{ij} \) in the original matrix \( A \) transforms into \( a_{ji} \) in \( A^{T} \).
- If \( A \) is of order \( m \times n \), then \( A^{T} \) is of order \( n \times m \).
- Transposing a matrix twice returns the original matrix, i.e., \((A^{T})^{T} = A\).
Symmetric Matrix
A symmetric matrix is a special kind of square matrix. It is equal to its own transpose, meaning for a matrix \( A \): \( A = A^{T} \).
- To be symmetric, a matrix must be square, i.e., have the same number of rows and columns.
- In practical terms, the element at the \( i \)-th row and \( j \)-th column must be equal to the element at the \( j \)-th row and \( i \)-th column.
- For example, the matrix \(\begin{bmatrix} 1 & 2 \ 2 & 1 \end{bmatrix}\) is symmetric, as swapping rows and columns does not change it.
Transpose Property
The transpose property is a useful rule when working with matrices. It states how operations with matrices behave under transposition. The property \((B+C)^{T} = B^{T} + C^{T}\) is particularly important.
- It tells us that the transpose of the sum of two matrices is the same as the sum of their transposes.
- This property can simplify calculations and is critical in proofs involving matrix symmetry.
- Another important transpose property is that \((BC)^{T} = C^{T}B^{T}\) for any matrices \( B \) and \( C \).
Square Matrix
A square matrix is a matrix with the same number of rows and columns. It's denoted as an \( n \times n \) matrix.
- Square matrices are central in linear algebra, as many important concepts, like determinants and eigenvalues, apply to them.
- Properties such as diagonalizability and being able to define an inverse are unique to square matrices.
- Square matrices can be symmetric, skew-symmetric, diagonal, or identity matrices, depending on their specific properties.