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Consider the following statements in res...

Consider the following statements in respect of symmetric matrices A and B.
I. AB is symmetric
II. `A^(2)+B^(2)` is symmetric
Which of the above statements is/are correct ?

A

A. Only I

B

B. Only II

C

C. Both I and II

D

D. Neither I nor II

Text Solution

AI Generated Solution

The correct Answer is:
To determine the correctness of the statements regarding symmetric matrices \( A \) and \( B \), we will analyze each statement step by step. ### Step 1: Understanding Symmetric Matrices A matrix \( A \) is called symmetric if it is equal to its transpose, i.e., \[ A^T = A \] Similarly, for matrix \( B \): \[ B^T = B \] ### Step 2: Analyzing Statement I: \( AB \) is symmetric To check if \( AB \) is symmetric, we need to find the transpose of the product \( AB \): \[ (AB)^T = B^T A^T \] Using the property of transposes, we know that \( (XY)^T = Y^T X^T \) for any matrices \( X \) and \( Y \). Since both \( A \) and \( B \) are symmetric, we can substitute: \[ (AB)^T = B A \] Now, for \( AB \) to be symmetric, we need: \[ (AB)^T = AB \] This means: \[ BA = AB \] However, this is not always true for arbitrary matrices \( A \) and \( B \). Therefore, we conclude that: \[ AB \text{ is not necessarily symmetric.} \] Thus, **Statement I is incorrect**. ### Step 3: Analyzing Statement II: \( A^2 + B^2 \) is symmetric To check if \( A^2 + B^2 \) is symmetric, we need to find the transpose of \( A^2 + B^2 \): \[ (A^2 + B^2)^T = A^2 + B^2 \] Using the property of transposes, we can write: \[ (A^2 + B^2)^T = A^2 + B^2 \] Now, we find the transpose of \( A^2 \) and \( B^2 \): \[ A^2 = A \cdot A \implies (A^2)^T = (A \cdot A)^T = A^T \cdot A^T = A \cdot A = A^2 \] Similarly for \( B^2 \): \[ B^2 = B \cdot B \implies (B^2)^T = (B \cdot B)^T = B^T \cdot B^T = B \cdot B = B^2 \] Thus, we have: \[ (A^2 + B^2)^T = A^2 + B^2 \] This shows that \( A^2 + B^2 \) is symmetric. Therefore, **Statement II is correct**. ### Conclusion From our analysis: - Statement I: \( AB \) is symmetric - **Incorrect** - Statement II: \( A^2 + B^2 \) is symmetric - **Correct** Thus, the correct answer is that only Statement II is correct. ### Final Answer Only Statement II is correct. ---
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