Linear Algebra Examples

⎡ ⎢ ⎣ \begin{matrix} 3 & 2 & 1 4 & 4 & 4 1 & 2 & 3 \end{matrix} ⎤ ⎥ ⎦

$[\begin{array}{c} 3 & 2 & 1 \\ 4 & 4 & 4 \\ 1 & 2 & 3 \end{array}]$

Step 1

Consider the corresponding sign chart.

⎡ ⎢ ⎣ \begin{matrix} + & - & + - & + & - + & - & + \end{matrix} ⎤ ⎥ ⎦

$[\begin{array}{c} + & - & + \\ - & + & - \\ + & - & + \end{array}]$

Step 2

Use the sign chart and the given matrix to find the cofactor of each element.

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Step 2.1

Calculate the minor for element

a_{11}

$a_{11}$ .

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Step 2.1.1

The minor for

a_{11}

$a_{11}$ is the determinant with row

1

$1$ and column

1

$1$ deleted.

∣ ∣ ∣ \begin{matrix} 4 & 4 2 & 3 \end{matrix} ∣ ∣ ∣

$| \begin{array}{c} 4 & 4 \\ 2 & 3 \end{array} |$

Step 2.1.2

Evaluate the determinant.

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Step 2.1.2.1

The determinant of a

2 \times 2

$2 \times 2$ matrix can be found using the formula

∣ ∣ ∣ \begin{matrix} a & b c & d \end{matrix} ∣ ∣ ∣ = a d - c b

$| \begin{array}{c} a & b \\ c & d \end{array} | = a d - c b$ .

a_{11} = 4 \cdot 3 - 2 \cdot 4

$a_{11} = 4 \cdot 3 - 2 \cdot 4$

Step 2.1.2.2

Simplify the determinant.

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Step 2.1.2.2.1

Simplify each term.

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Step 2.1.2.2.1.1

Multiply

4

$4$ by

3

$3$ .

a_{11} = 12 - 2 \cdot 4

$a_{11} = 12 - 2 \cdot 4$

Step 2.1.2.2.1.2

Multiply

- 2

$- 2$ by

4

$4$ .

a_{11} = 12 - 8

$a_{11} = 12 - 8$

a_{11} = 12 - 8

$a_{11} = 12 - 8$

Step 2.1.2.2.2

Subtract

8

$8$ from

12

$12$ .

a_{11} = 4

$a_{11} = 4$

a_{11} = 4

$a_{11} = 4$

a_{11} = 4

$a_{11} = 4$

a_{11} = 4

$a_{11} = 4$

Step 2.2

Calculate the minor for element

a_{12}

$a_{12}$ .

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Step 2.2.1

The minor for

a_{12}

$a_{12}$ is the determinant with row

1

$1$ and column

2

$2$ deleted.

∣ ∣ ∣ \begin{matrix} 4 & 4 1 & 3 \end{matrix} ∣ ∣ ∣

$| \begin{array}{c} 4 & 4 \\ 1 & 3 \end{array} |$

Step 2.2.2

Evaluate the determinant.

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Step 2.2.2.1

The determinant of a

2 \times 2

$2 \times 2$ matrix can be found using the formula

∣ ∣ ∣ \begin{matrix} a & b c & d \end{matrix} ∣ ∣ ∣ = a d - c b

$| \begin{array}{c} a & b \\ c & d \end{array} | = a d - c b$ .

a_{12} = 4 \cdot 3 - 1 \cdot 4

$a_{12} = 4 \cdot 3 - 1 \cdot 4$

Step 2.2.2.2

Simplify the determinant.

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Step 2.2.2.2.1

Simplify each term.

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Step 2.2.2.2.1.1

Multiply

4

$4$ by

3

$3$ .

a_{12} = 12 - 1 \cdot 4

$a_{12} = 12 - 1 \cdot 4$

Step 2.2.2.2.1.2

Multiply

- 1

$- 1$ by

4

$4$ .

a_{12} = 12 - 4

$a_{12} = 12 - 4$

a_{12} = 12 - 4

$a_{12} = 12 - 4$

Step 2.2.2.2.2

Subtract

4

$4$ from

12

$12$ .

a_{12} = 8

$a_{12} = 8$

a_{12} = 8

$a_{12} = 8$

a_{12} = 8

$a_{12} = 8$

a_{12} = 8

$a_{12} = 8$

Step 2.3

Calculate the minor for element

a_{13}

$a_{13}$ .

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Step 2.3.1

The minor for

a_{13}

$a_{13}$ is the determinant with row

1

$1$ and column

3

$3$ deleted.

∣ ∣ ∣ \begin{matrix} 4 & 4 1 & 2 \end{matrix} ∣ ∣ ∣

$| \begin{array}{c} 4 & 4 \\ 1 & 2 \end{array} |$

Step 2.3.2

Evaluate the determinant.

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Step 2.3.2.1

The determinant of a

2 \times 2

$2 \times 2$ matrix can be found using the formula

∣ ∣ ∣ \begin{matrix} a & b c & d \end{matrix} ∣ ∣ ∣ = a d - c b

$| \begin{array}{c} a & b \\ c & d \end{array} | = a d - c b$ .

a_{13} = 4 \cdot 2 - 1 \cdot 4

$a_{13} = 4 \cdot 2 - 1 \cdot 4$

Step 2.3.2.2

Simplify the determinant.

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Step 2.3.2.2.1

Simplify each term.

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Step 2.3.2.2.1.1

Multiply

4

$4$ by

2

$2$ .

a_{13} = 8 - 1 \cdot 4

$a_{13} = 8 - 1 \cdot 4$

Step 2.3.2.2.1.2

Multiply

- 1

$- 1$ by

4

$4$ .

a_{13} = 8 - 4

$a_{13} = 8 - 4$

a_{13} = 8 - 4

$a_{13} = 8 - 4$

Step 2.3.2.2.2

Subtract

4

$4$ from

8

$8$ .

a_{13} = 4

$a_{13} = 4$

a_{13} = 4

$a_{13} = 4$

a_{13} = 4

$a_{13} = 4$

a_{13} = 4

$a_{13} = 4$

Step 2.4

Calculate the minor for element

a_{21}

$a_{21}$ .

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Step 2.4.1

The minor for

a_{21}

$a_{21}$ is the determinant with row

2

$2$ and column

1

$1$ deleted.

∣ ∣ ∣ \begin{matrix} 2 & 1 2 & 3 \end{matrix} ∣ ∣ ∣

$| \begin{array}{c} 2 & 1 \\ 2 & 3 \end{array} |$

Step 2.4.2

Evaluate the determinant.

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Step 2.4.2.1

The determinant of a

2 \times 2

$2 \times 2$ matrix can be found using the formula

∣ ∣ ∣ \begin{matrix} a & b c & d \end{matrix} ∣ ∣ ∣ = a d - c b

$| \begin{array}{c} a & b \\ c & d \end{array} | = a d - c b$ .

a_{21} = 2 \cdot 3 - 2 \cdot 1

$a_{21} = 2 \cdot 3 - 2 \cdot 1$

Step 2.4.2.2

Simplify the determinant.

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Step 2.4.2.2.1

Simplify each term.

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Step 2.4.2.2.1.1

Multiply

2

$2$ by

3

$3$ .

a_{21} = 6 - 2 \cdot 1

$a_{21} = 6 - 2 \cdot 1$

Step 2.4.2.2.1.2

Multiply

- 2

$- 2$ by

1

$1$ .

a_{21} = 6 - 2

$a_{21} = 6 - 2$

a_{21} = 6 - 2

$a_{21} = 6 - 2$

Step 2.4.2.2.2

Subtract

2

$2$ from

6

$6$ .

a_{21} = 4

$a_{21} = 4$

a_{21} = 4

$a_{21} = 4$

a_{21} = 4

$a_{21} = 4$

a_{21} = 4

$a_{21} = 4$

Step 2.5

Calculate the minor for element

a_{22}

$a_{22}$ .

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Step 2.5.1

The minor for

a_{22}

$a_{22}$ is the determinant with row

2

$2$ and column

2

$2$ deleted.

∣ ∣ ∣ \begin{matrix} 3 & 1 1 & 3 \end{matrix} ∣ ∣ ∣

$| \begin{array}{c} 3 & 1 \\ 1 & 3 \end{array} |$

Step 2.5.2

Evaluate the determinant.

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Step 2.5.2.1

The determinant of a

2 \times 2

$2 \times 2$ matrix can be found using the formula

∣ ∣ ∣ \begin{matrix} a & b c & d \end{matrix} ∣ ∣ ∣ = a d - c b

$| \begin{array}{c} a & b \\ c & d \end{array} | = a d - c b$ .

a_{22} = 3 \cdot 3 - 1 \cdot 1

$a_{22} = 3 \cdot 3 - 1 \cdot 1$

Step 2.5.2.2

Simplify the determinant.

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Step 2.5.2.2.1

Simplify each term.

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Step 2.5.2.2.1.1

Multiply

3

$3$ by

3

$3$ .

a_{22} = 9 - 1 \cdot 1

$a_{22} = 9 - 1 \cdot 1$

Step 2.5.2.2.1.2

Multiply

- 1

$- 1$ by

1

$1$ .

a_{22} = 9 - 1

$a_{22} = 9 - 1$

a_{22} = 9 - 1

$a_{22} = 9 - 1$

Step 2.5.2.2.2

Subtract

1

$1$ from

9

$9$ .

a_{22} = 8

$a_{22} = 8$

a_{22} = 8

$a_{22} = 8$

a_{22} = 8

$a_{22} = 8$

a_{22} = 8

$a_{22} = 8$

Step 2.6

Calculate the minor for element

a_{23}

$a_{23}$ .

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Step 2.6.1

The minor for

a_{23}

$a_{23}$ is the determinant with row

2

$2$ and column

3

$3$ deleted.

∣ ∣ ∣ \begin{matrix} 3 & 2 1 & 2 \end{matrix} ∣ ∣ ∣

$| \begin{array}{c} 3 & 2 \\ 1 & 2 \end{array} |$

Step 2.6.2

Evaluate the determinant.

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Step 2.6.2.1

The determinant of a

2 \times 2

$2 \times 2$ matrix can be found using the formula

∣ ∣ ∣ \begin{matrix} a & b c & d \end{matrix} ∣ ∣ ∣ = a d - c b

$| \begin{array}{c} a & b \\ c & d \end{array} | = a d - c b$ .

a_{23} = 3 \cdot 2 - 1 \cdot 2

$a_{23} = 3 \cdot 2 - 1 \cdot 2$

Step 2.6.2.2

Simplify the determinant.

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Step 2.6.2.2.1

Simplify each term.

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Step 2.6.2.2.1.1

Multiply

3

$3$ by

2

$2$ .

a_{23} = 6 - 1 \cdot 2

$a_{23} = 6 - 1 \cdot 2$

Step 2.6.2.2.1.2

Multiply

- 1

$- 1$ by

2

$2$ .

a_{23} = 6 - 2

$a_{23} = 6 - 2$

a_{23} = 6 - 2

$a_{23} = 6 - 2$

Step 2.6.2.2.2

Subtract

2

$2$ from

6

$6$ .

a_{23} = 4

$a_{23} = 4$

a_{23} = 4

$a_{23} = 4$

a_{23} = 4

$a_{23} = 4$

a_{23} = 4

$a_{23} = 4$

Step 2.7

Calculate the minor for element

a_{31}

$a_{31}$ .

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Step 2.7.1

The minor for

a_{31}

$a_{31}$ is the determinant with row

3

$3$ and column

1

$1$ deleted.

∣ ∣ ∣ \begin{matrix} 2 & 1 4 & 4 \end{matrix} ∣ ∣ ∣

$| \begin{array}{c} 2 & 1 \\ 4 & 4 \end{array} |$

Step 2.7.2

Evaluate the determinant.

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Step 2.7.2.1

The determinant of a

2 \times 2

$2 \times 2$ matrix can be found using the formula

∣ ∣ ∣ \begin{matrix} a & b c & d \end{matrix} ∣ ∣ ∣ = a d - c b

$| \begin{array}{c} a & b \\ c & d \end{array} | = a d - c b$ .

a_{31} = 2 \cdot 4 - 4 \cdot 1

$a_{31} = 2 \cdot 4 - 4 \cdot 1$

Step 2.7.2.2

Simplify the determinant.

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Step 2.7.2.2.1

Simplify each term.

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Step 2.7.2.2.1.1

Multiply

2

$2$ by

4

$4$ .

a_{31} = 8 - 4 \cdot 1

$a_{31} = 8 - 4 \cdot 1$

Step 2.7.2.2.1.2

Multiply

- 4

$- 4$ by

1

$1$ .

a_{31} = 8 - 4

$a_{31} = 8 - 4$

a_{31} = 8 - 4

$a_{31} = 8 - 4$

Step 2.7.2.2.2

Subtract

4

$4$ from

8

$8$ .

a_{31} = 4

$a_{31} = 4$

a_{31} = 4

$a_{31} = 4$

a_{31} = 4

$a_{31} = 4$

a_{31} = 4

$a_{31} = 4$

Step 2.8

Calculate the minor for element

a_{32}

$a_{32}$ .

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Step 2.8.1

The minor for

a_{32}

$a_{32}$ is the determinant with row

3

$3$ and column

2

$2$ deleted.

∣ ∣ ∣ \begin{matrix} 3 & 1 4 & 4 \end{matrix} ∣ ∣ ∣

$| \begin{array}{c} 3 & 1 \\ 4 & 4 \end{array} |$

Step 2.8.2

Evaluate the determinant.

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Step 2.8.2.1

The determinant of a

2 \times 2

$2 \times 2$ matrix can be found using the formula

∣ ∣ ∣ \begin{matrix} a & b c & d \end{matrix} ∣ ∣ ∣ = a d - c b

$| \begin{array}{c} a & b \\ c & d \end{array} | = a d - c b$ .

a_{32} = 3 \cdot 4 - 4 \cdot 1

$a_{32} = 3 \cdot 4 - 4 \cdot 1$

Step 2.8.2.2

Simplify the determinant.

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Step 2.8.2.2.1

Simplify each term.

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Step 2.8.2.2.1.1

Multiply

3

$3$ by

4

$4$ .

a_{32} = 12 - 4 \cdot 1

$a_{32} = 12 - 4 \cdot 1$

Step 2.8.2.2.1.2

Multiply

- 4

$- 4$ by

1

$1$ .

a_{32} = 12 - 4

$a_{32} = 12 - 4$

a_{32} = 12 - 4

$a_{32} = 12 - 4$

Step 2.8.2.2.2

Subtract

4

$4$ from

12

$12$ .

a_{32} = 8

$a_{32} = 8$

a_{32} = 8

$a_{32} = 8$

a_{32} = 8

$a_{32} = 8$

a_{32} = 8

$a_{32} = 8$

Step 2.9

Calculate the minor for element

a_{33}

$a_{33}$ .

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Step 2.9.1

The minor for

a_{33}

$a_{33}$ is the determinant with row

3

$3$ and column

3

$3$ deleted.

∣ ∣ ∣ \begin{matrix} 3 & 2 4 & 4 \end{matrix} ∣ ∣ ∣

$| \begin{array}{c} 3 & 2 \\ 4 & 4 \end{array} |$

Step 2.9.2

Evaluate the determinant.

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Step 2.9.2.1

The determinant of a

2 \times 2

$2 \times 2$ matrix can be found using the formula

∣ ∣ ∣ \begin{matrix} a & b c & d \end{matrix} ∣ ∣ ∣ = a d - c b

$| \begin{array}{c} a & b \\ c & d \end{array} | = a d - c b$ .

a_{33} = 3 \cdot 4 - 4 \cdot 2

$a_{33} = 3 \cdot 4 - 4 \cdot 2$

Step 2.9.2.2

Simplify the determinant.

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Step 2.9.2.2.1

Simplify each term.

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Step 2.9.2.2.1.1

Multiply

3

$3$ by

4

$4$ .

a_{33} = 12 - 4 \cdot 2

$a_{33} = 12 - 4 \cdot 2$

Step 2.9.2.2.1.2

Multiply

- 4

$- 4$ by

2

$2$ .

a_{33} = 12 - 8

$a_{33} = 12 - 8$

a_{33} = 12 - 8

$a_{33} = 12 - 8$

Step 2.9.2.2.2

Subtract

8

$8$ from

12

$12$ .

a_{33} = 4

$a_{33} = 4$

a_{33} = 4

$a_{33} = 4$

a_{33} = 4

$a_{33} = 4$

a_{33} = 4

$a_{33} = 4$

Step 2.10

The cofactor matrix is a matrix of the minors with the sign changed for the elements in the

-

$-$ positions on the sign chart.

⎡ ⎢ ⎣ \begin{matrix} 4 & - 8 & 4 - 4 & 8 & - 4 4 & - 8 & 4 \end{matrix} ⎤ ⎥ ⎦

$[\begin{array}{c} 4 & - 8 & 4 \\ - 4 & 8 & - 4 \\ 4 & - 8 & 4 \end{array}]$

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