Symmetric Matrix

Rarely Tested

Symmetric Matrix

## Definition / Concept

A square matrix $$a$$ is symmetric if:

$$A^T=A$$

Therefore:

$$a_{ij}=a_{ji}$$

## Usage

- Used when corresponding elements about the principal diagonal are equal.

Question 1

Let $$A$$ be a $$3 \times 3$$ symmetric matrix ($$A^T = A$$) and $$B$$ be a $$3 \times 3$$ skew-symmetric matrix ($$B^T = -B$$) such that:

$$A^4 + B^4 = A$$

Define the following matrices:

  • $$P = (A - B)^{2023} (A + B)^{2023} - (A + B)^{2023} (A - B)^{2023}$$
  • $$Q = (A + B)^T (A^4 + B^4) (A - B) + (A - B)^T (A^4 + B^4) (A + B)$$

Statements:

  • S1: $$P$$ is a skew-symmetric matrix.
  • S2: $$Q$$ is a symmetric matrix.
Question 2

Let $$A$$ and $$B$$ be two square matrices of order $$3 \times 3$$ with real entries. If the matrix $$A$$ is symmetric such that $$A^T = A$$, and the matrix $$B$$ is skew-symmetric such that $$B^T = -B$$, then the value of the determinant of the matrix $$A B + B A$$ is :

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