1. Which of the following sets of matrices form a subspace of M22? (assume a, b range across the real numbers) a b] А. 6 a [a 1 a С. 0 [o o] D. 0 0 B.

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### Linear Algebra: Subspaces of Matrices

**Question:**
Which of the following sets of matrices form a subspace of \( M_{2,2} \)? (assume \( a, b \) range across the real numbers)

**Options:**
A. 
\[
\begin{bmatrix}
a & b \\
b & a
\end{bmatrix}
\]

B. 
\[
\begin{bmatrix}
a & 1 \\
1 & a
\end{bmatrix}
\]

C. 
\[
\begin{bmatrix}
a & 0 \\
0 & a
\end{bmatrix}
\]

D.
\[
\begin{bmatrix}
0 & 0 \\
0 & 0
\end{bmatrix}
\]

**Explanation:**

- **Option A** presents symmetric matrices, where the off-diagonal elements are equal.
  
- **Option B** presents a specific type of matrix where the off-diagonal elements are always 1.

- **Option C** presents diagonal matrices with equal elements on the diagonal.

- **Option D** presents the zero matrix, which is the identity element of matrix addition.

**Discussion:**

For a set of matrices to form a subspace of \( M_{2,2} \), it must be closed under matrix addition and scalar multiplication, and it must contain the zero matrix.

- **Option A** (Symmetric Matrices): These matrices form a subspace because the set of symmetric matrices is closed under addition and scalar multiplication.
  
- **Option B** (Matrices with fixed off-diagonal elements of 1): This set does not form a subspace because it is not closed under scalar multiplication (scaling the matrix would not keep the off-diagonal elements as 1).

- **Option C** (Diagonal Matrices with equal diagonal elements): These matrices form a subspace as they are closed under addition and scalar multiplication.

- **Option D** (Zero Matrix): This set does form a subspace as the set containing only the zero matrix trivially satisfies all the subspace properties.

**Answer:**
A and C
Transcribed Image Text:### Linear Algebra: Subspaces of Matrices **Question:** Which of the following sets of matrices form a subspace of \( M_{2,2} \)? (assume \( a, b \) range across the real numbers) **Options:** A. \[ \begin{bmatrix} a & b \\ b & a \end{bmatrix} \] B. \[ \begin{bmatrix} a & 1 \\ 1 & a \end{bmatrix} \] C. \[ \begin{bmatrix} a & 0 \\ 0 & a \end{bmatrix} \] D. \[ \begin{bmatrix} 0 & 0 \\ 0 & 0 \end{bmatrix} \] **Explanation:** - **Option A** presents symmetric matrices, where the off-diagonal elements are equal. - **Option B** presents a specific type of matrix where the off-diagonal elements are always 1. - **Option C** presents diagonal matrices with equal elements on the diagonal. - **Option D** presents the zero matrix, which is the identity element of matrix addition. **Discussion:** For a set of matrices to form a subspace of \( M_{2,2} \), it must be closed under matrix addition and scalar multiplication, and it must contain the zero matrix. - **Option A** (Symmetric Matrices): These matrices form a subspace because the set of symmetric matrices is closed under addition and scalar multiplication. - **Option B** (Matrices with fixed off-diagonal elements of 1): This set does not form a subspace because it is not closed under scalar multiplication (scaling the matrix would not keep the off-diagonal elements as 1). - **Option C** (Diagonal Matrices with equal diagonal elements): These matrices form a subspace as they are closed under addition and scalar multiplication. - **Option D** (Zero Matrix): This set does form a subspace as the set containing only the zero matrix trivially satisfies all the subspace properties. **Answer:** A and C
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