Precalculus Examples

$[\begin{array}{c} 12 & 23 & - 6 \\ 11 & - 8 & 13 \\ 7 & 21 & 15 \end{array}]$

Step 1

Consider the corresponding sign chart.

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

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 $a_{11}$ .

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Step 2.1.1

The minor for $a_{11}$ is the determinant with row $1$ and column $1$ deleted.

$| \begin{array}{c} - 8 & 13 \\ 21 & 15 \end{array} |$

Step 2.1.2

Evaluate the determinant.

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Step 2.1.2.1

The determinant of a $2 \times 2$ matrix can be found using the formula $| \begin{array}{c} a & b \\ c & d \end{array} | = a d - c b$ .

$a_{11} = - 8 \cdot 15 - 21 \cdot 13$

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 $- 8$ by $15$ .

$a_{11} = - 120 - 21 \cdot 13$

Step 2.1.2.2.1.2

Multiply $- 21$ by $13$ .

$a_{11} = - 120 - 273$

Step 2.1.2.2.2

Subtract $273$ from $- 120$ .

$a_{11} = - 393$

Step 2.2

Calculate the minor for element $a_{12}$ .

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Step 2.2.1

The minor for $a_{12}$ is the determinant with row $1$ and column $2$ deleted.

$| \begin{array}{c} 11 & 13 \\ 7 & 15 \end{array} |$

Step 2.2.2

Evaluate the determinant.

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Step 2.2.2.1

The determinant of a $2 \times 2$ matrix can be found using the formula $| \begin{array}{c} a & b \\ c & d \end{array} | = a d - c b$ .

$a_{12} = 11 \cdot 15 - 7 \cdot 13$

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 $11$ by $15$ .

$a_{12} = 165 - 7 \cdot 13$

Step 2.2.2.2.1.2

Multiply $- 7$ by $13$ .

$a_{12} = 165 - 91$

Step 2.2.2.2.2

Subtract $91$ from $165$ .

$a_{12} = 74$

Step 2.3

Calculate the minor for element $a_{13}$ .

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Step 2.3.1

The minor for $a_{13}$ is the determinant with row $1$ and column $3$ deleted.

$| \begin{array}{c} 11 & - 8 \\ 7 & 21 \end{array} |$

Step 2.3.2

Evaluate the determinant.

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Step 2.3.2.1

The determinant of a $2 \times 2$ matrix can be found using the formula $| \begin{array}{c} a & b \\ c & d \end{array} | = a d - c b$ .

$a_{13} = 11 \cdot 21 - 7 \cdot - 8$

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 $11$ by $21$ .

$a_{13} = 231 - 7 \cdot - 8$

Step 2.3.2.2.1.2

Multiply $- 7$ by $- 8$ .

$a_{13} = 231 + 56$

Step 2.3.2.2.2

Add $231$ and $56$ .

$a_{13} = 287$

Step 2.4

Calculate the minor for element $a_{21}$ .

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Step 2.4.1

The minor for $a_{21}$ is the determinant with row $2$ and column $1$ deleted.

$| \begin{array}{c} 23 & - 6 \\ 21 & 15 \end{array} |$

Step 2.4.2

Evaluate the determinant.

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Step 2.4.2.1

The determinant of a $2 \times 2$ matrix can be found using the formula $| \begin{array}{c} a & b \\ c & d \end{array} | = a d - c b$ .

$a_{21} = 23 \cdot 15 - 21 \cdot - 6$

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 $23$ by $15$ .

$a_{21} = 345 - 21 \cdot - 6$

Step 2.4.2.2.1.2

Multiply $- 21$ by $- 6$ .

$a_{21} = 345 + 126$

Step 2.4.2.2.2

Add $345$ and $126$ .

$a_{21} = 471$

Step 2.5

Calculate the minor for element $a_{22}$ .

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Step 2.5.1

The minor for $a_{22}$ is the determinant with row $2$ and column $2$ deleted.

$| \begin{array}{c} 12 & - 6 \\ 7 & 15 \end{array} |$

Step 2.5.2

Evaluate the determinant.

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Step 2.5.2.1

The determinant of a $2 \times 2$ matrix can be found using the formula $| \begin{array}{c} a & b \\ c & d \end{array} | = a d - c b$ .

$a_{22} = 12 \cdot 15 - 7 \cdot - 6$

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 $12$ by $15$ .

$a_{22} = 180 - 7 \cdot - 6$

Step 2.5.2.2.1.2

Multiply $- 7$ by $- 6$ .

$a_{22} = 180 + 42$

Step 2.5.2.2.2

Add $180$ and $42$ .

$a_{22} = 222$

Step 2.6

Calculate the minor for element $a_{23}$ .

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Step 2.6.1

The minor for $a_{23}$ is the determinant with row $2$ and column $3$ deleted.

$| \begin{array}{c} 12 & 23 \\ 7 & 21 \end{array} |$

Step 2.6.2

Evaluate the determinant.

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Step 2.6.2.1

The determinant of a $2 \times 2$ matrix can be found using the formula $| \begin{array}{c} a & b \\ c & d \end{array} | = a d - c b$ .

$a_{23} = 12 \cdot 21 - 7 \cdot 23$

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 $12$ by $21$ .

$a_{23} = 252 - 7 \cdot 23$

Step 2.6.2.2.1.2

Multiply $- 7$ by $23$ .

$a_{23} = 252 - 161$

Step 2.6.2.2.2

Subtract $161$ from $252$ .

$a_{23} = 91$

Step 2.7

Calculate the minor for element $a_{31}$ .

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Step 2.7.1

The minor for $a_{31}$ is the determinant with row $3$ and column $1$ deleted.

$| \begin{array}{c} 23 & - 6 \\ - 8 & 13 \end{array} |$

Step 2.7.2

Evaluate the determinant.

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Step 2.7.2.1

The determinant of a $2 \times 2$ matrix can be found using the formula $| \begin{array}{c} a & b \\ c & d \end{array} | = a d - c b$ .

$a_{31} = 23 \cdot 13 - (- 8 \cdot - 6)$

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 $23$ by $13$ .

$a_{31} = 299 - (- 8 \cdot - 6)$

Step 2.7.2.2.1.2

Multiply $- (- 8 \cdot - 6)$ .

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Step 2.7.2.2.1.2.1

Multiply $- 8$ by $- 6$ .

$a_{31} = 299 - 1 \cdot 48$

Step 2.7.2.2.1.2.2

Multiply $- 1$ by $48$ .

$a_{31} = 299 - 48$

Step 2.7.2.2.2

Subtract $48$ from $299$ .

$a_{31} = 251$

Step 2.8

Calculate the minor for element $a_{32}$ .

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Step 2.8.1

The minor for $a_{32}$ is the determinant with row $3$ and column $2$ deleted.

$| \begin{array}{c} 12 & - 6 \\ 11 & 13 \end{array} |$

Step 2.8.2

Evaluate the determinant.

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Step 2.8.2.1

The determinant of a $2 \times 2$ matrix can be found using the formula $| \begin{array}{c} a & b \\ c & d \end{array} | = a d - c b$ .

$a_{32} = 12 \cdot 13 - 11 \cdot - 6$

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 $12$ by $13$ .

$a_{32} = 156 - 11 \cdot - 6$

Step 2.8.2.2.1.2

Multiply $- 11$ by $- 6$ .

$a_{32} = 156 + 66$

Step 2.8.2.2.2

Add $156$ and $66$ .

$a_{32} = 222$

Step 2.9

Calculate the minor for element $a_{33}$ .

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Step 2.9.1

The minor for $a_{33}$ is the determinant with row $3$ and column $3$ deleted.

$| \begin{array}{c} 12 & 23 \\ 11 & - 8 \end{array} |$

Step 2.9.2

Evaluate the determinant.

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Step 2.9.2.1

The determinant of a $2 \times 2$ matrix can be found using the formula $| \begin{array}{c} a & b \\ c & d \end{array} | = a d - c b$ .

$a_{33} = 12 \cdot - 8 - 11 \cdot 23$

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 $12$ by $- 8$ .

$a_{33} = - 96 - 11 \cdot 23$

Step 2.9.2.2.1.2

Multiply $- 11$ by $23$ .

$a_{33} = - 96 - 253$

Step 2.9.2.2.2

Subtract $253$ from $- 96$ .

$a_{33} = - 349$

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.

$[\begin{array}{c} - 393 & - 74 & 287 \\ - 471 & 222 & - 91 \\ 251 & - 222 & - 349 \end{array}]$