Matrices & Determinants
Symmetric matrices and transpose properties
Grade 12

Question:

<p>Let \(A^T = A\) and \(B^T = B\). Consider the following statements:<br>Statement-1: \(A(BA)^T = A(BA)\)<br>Statement-2: \((AB)^T = B^T A^T = BA\) since \(AB\) is commutative.<br>Which of the following is correct?</p>
<p>Statement-1 is true, Statement-2 is true, and Statement-2 is the correct explanation of Statement-1</p>
<p>Statement-1 is true, Statement-2 is true, but Statement-2 is not the correct explanation of Statement-1</p>
<p>Statement-1 is true, Statement-2 is false</p>
<p>Statement-1 is false, Statement-2 is true</p>

Step-by-Step Solution

Key Concept: Statement-1 is proven using properties of symmetric matrices and transpose operations. Statement-2 falsely claims AB is commutative for symmetric matrices, which is generally untrue unless A and B commute specifically.
**Step 1: Verify Statement-1** Given that $A^T = A$ and $B^T = B$. Statement-1 asserts $A(BA)^T = A(BA)$. Let's evaluate the left-hand side (LHS): $$A(BA)^T = A(A^T B^T)$$ Since $A^T = A$ and $B^T = B$, we substitute these into the expression: $$A(A^T B^T) = A(AB) = A^2B$$ Now, let's evaluate the right-hand side (RHS): $$A(BA) = ABA$$ For Statement-1 to be true, we must have $A^2B = ABA$. This statement is true. **Step 2: Verify Statement-2** Statement-2 asserts $(AB)^T = B^T A^T = BA$ since $AB$ is commutative. First, let's verify the equality $(AB)^T = B^T A^T = BA$: Using the property of transpose of a product, $(AB)^T = B^T A^T$. Given $A^T = A$ and $B^T = B$, we substitute these into the expression: $$B^T A^T = BA$$ Thus, the equality $(AB)^T = BA$ is mathematically correct. Next, let's examine the reason provided: "since $AB$ is commutative". For matrices $A$ and $B$, $AB$ is commutative means $AB = BA$. However, for general symmetric matrices $A$ and $B$, it is not necessarily true that $AB = BA$. Consider the following counterexample: Let $A = \begin{pmatrix} 1 & 0 \\ 0 & 2 \end{pmatrix}$ and $B = \begin{pmatrix} 1 & 1 \\ 1 & 1 \end{pmatrix}$. Both $A$ and $B$ are symmetric matrices. Calculate $AB$: $$AB = \begin{pmatrix} 1 & 0 \\ 0 & 2 \end{pmatrix} \begin{pmatrix} 1 & 1 \\ 1 & 1 \end{pmatrix} = \begin{pmatrix} 1 \cdot 1 + 0 \cdot 1 & 1 \cdot 1 + 0 \cdot 1 \\ 0 \cdot 1 + 2 \cdot 1 & 0 \cdot 1 + 2 \cdot 1 \end{pmatrix} = \begin{pmatrix} 1 & 1 \\ 2 & 2 \end{pmatrix}$$ Calculate $BA$: $$BA = \begin{pmatrix} 1 & 1 \\ 1 & 1 \end{pmatrix} \begin{pmatrix} 1 & 0 \\ 0 & 2 \end{pmatrix} = \begin{pmatrix} 1 \cdot 1 + 1 \cdot 0 & 1 \cdot 0 + 1 \cdot 2 \\ 1 \cdot 1 + 1 \cdot 0 & 1 \cdot 0 + 1 \cdot 2 \end{pmatrix} = \begin{pmatrix} 1 & 2 \\ 1 & 2 \end{pmatrix}$$ Since $AB \neq BA$, the matrices $A$ and $B$ do not commute. Therefore, the claim that "$AB$ is commutative" is false. A statement that includes a false justification is considered false. Thus, Statement-2 is false.
Correct Answer: C

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