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