In this problem we need to find the dimensions of Nul A. Given A is 4x7 matrix and it has three pivot columns.

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### Finding the Nullity of Matrix A

**Problem Statement:**  
In this problem, we need to find the dimensions of the null space (Nul A).

**Given:**
- \( A \) is a \( 4 \times 7 \) matrix and it has three pivot columns.

Clearly, \( A \) has four rows and seven columns.

\[ \text{col}(A) = \mathbb{R}^3 \]

**Explanation:**
- \( A \) has 7 columns and 3 pivot columns, so \( \text{col}(A) \) is a three-dimensional subspace of \( \mathbb{R}^4 \).
- Given \( A \) has 4 rows, \( \text{col}(A) \) is a subspace of \( \mathbb{R}^4 \), so it cannot equal \( \mathbb{R}^3 \).

The dimension of the column space of \( A \) is the rank of \( A \). Thus, \(\text{rank}(A) = 3\).

**Application of the Rank-Nullity Theorem:**
By using the Rank-Nullity Theorem:

\[ n = \text{null}(A) + \text{rank}(A) \]

Substituting the given values:

\[ \Rightarrow 7 = \text{null}(A) + 3 \]

Solving for \(\text{null}(A)\):

\[ \Rightarrow 7 - 3 = \text{null}(A) \]

Therefore, \(\text{null}(A) = 4\).

**Rank-Nullity Theorem Description:**
- Let \( A \) be an \( m \times n \) matrix,
- The theorem states: \[ n = \text{null}(A) + \text{rank}(A) \]
- \( n \) is the number of columns of \( A \).

This was the step-by-step process to find the nullity of matrix \( A \).
Transcribed Image Text:### Finding the Nullity of Matrix A **Problem Statement:** In this problem, we need to find the dimensions of the null space (Nul A). **Given:** - \( A \) is a \( 4 \times 7 \) matrix and it has three pivot columns. Clearly, \( A \) has four rows and seven columns. \[ \text{col}(A) = \mathbb{R}^3 \] **Explanation:** - \( A \) has 7 columns and 3 pivot columns, so \( \text{col}(A) \) is a three-dimensional subspace of \( \mathbb{R}^4 \). - Given \( A \) has 4 rows, \( \text{col}(A) \) is a subspace of \( \mathbb{R}^4 \), so it cannot equal \( \mathbb{R}^3 \). The dimension of the column space of \( A \) is the rank of \( A \). Thus, \(\text{rank}(A) = 3\). **Application of the Rank-Nullity Theorem:** By using the Rank-Nullity Theorem: \[ n = \text{null}(A) + \text{rank}(A) \] Substituting the given values: \[ \Rightarrow 7 = \text{null}(A) + 3 \] Solving for \(\text{null}(A)\): \[ \Rightarrow 7 - 3 = \text{null}(A) \] Therefore, \(\text{null}(A) = 4\). **Rank-Nullity Theorem Description:** - Let \( A \) be an \( m \times n \) matrix, - The theorem states: \[ n = \text{null}(A) + \text{rank}(A) \] - \( n \) is the number of columns of \( A \). This was the step-by-step process to find the nullity of matrix \( A \).
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