Algebra Examples

⎡ ⎢ ⎣ \begin{matrix} 1 & 2 & 3 4 & 5 & 6 7 & 8 & 9 \end{matrix} ⎤ ⎥ ⎦

$[\begin{array}{c} 1 & 2 & 3 \\ 4 & 5 & 6 \\ 7 & 8 & 9 \end{array}]$

Step 1

Choose the row or column with the most

0

$0$ elements. If there are no

0

$0$ elements choose any row or column. Multiply every element in row

1

$1$ by its cofactor and add.

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Step 1.1

Consider the corresponding sign chart.

∣ ∣ ∣ ∣ \begin{matrix} + & - & + - & + & - + & - & + \end{matrix} ∣ ∣ ∣ ∣

$| \begin{array}{c} + & - & + \\ - & + & - \\ + & - & + \end{array} |$

Step 1.2

The cofactor is the minor with the sign changed if the indices match a

-

$-$ position on the sign chart.

Step 1.3

The minor for

a_{11}

$a_{11}$ is the determinant with row

1

$1$ and column

1

$1$ deleted.

∣ ∣ ∣ \begin{matrix} 5 & 6 8 & 9 \end{matrix} ∣ ∣ ∣

$| \begin{array}{c} 5 & 6 \\ 8 & 9 \end{array} |$

Step 1.4

Multiply element

a_{11}

$a_{11}$ by its cofactor.

1 ∣ ∣ ∣ \begin{matrix} 5 & 6 8 & 9 \end{matrix} ∣ ∣ ∣

$1 | \begin{array}{c} 5 & 6 \\ 8 & 9 \end{array} |$

Step 1.5

The minor for

a_{12}

$a_{12}$ is the determinant with row

1

$1$ and column

2

$2$ deleted.

∣ ∣ ∣ \begin{matrix} 4 & 6 7 & 9 \end{matrix} ∣ ∣ ∣

$| \begin{array}{c} 4 & 6 \\ 7 & 9 \end{array} |$

Step 1.6

Multiply element

a_{12}

$a_{12}$ by its cofactor.

- 2 ∣ ∣ ∣ \begin{matrix} 4 & 6 7 & 9 \end{matrix} ∣ ∣ ∣

$- 2 | \begin{array}{c} 4 & 6 \\ 7 & 9 \end{array} |$

Step 1.7

The minor for

a_{13}

$a_{13}$ is the determinant with row

1

$1$ and column

3

$3$ deleted.

∣ ∣ ∣ \begin{matrix} 4 & 5 7 & 8 \end{matrix} ∣ ∣ ∣

$| \begin{array}{c} 4 & 5 \\ 7 & 8 \end{array} |$

Step 1.8

Multiply element

a_{13}

$a_{13}$ by its cofactor.

3 ∣ ∣ ∣ \begin{matrix} 4 & 5 7 & 8 \end{matrix} ∣ ∣ ∣

$3 | \begin{array}{c} 4 & 5 \\ 7 & 8 \end{array} |$

Step 1.9

Add the terms together.

1 ∣ ∣ ∣ \begin{matrix} 5 & 6 8 & 9 \end{matrix} ∣ ∣ ∣ - 2 ∣ ∣ ∣ \begin{matrix} 4 & 6 7 & 9 \end{matrix} ∣ ∣ ∣ + 3 ∣ ∣ ∣ \begin{matrix} 4 & 5 7 & 8 \end{matrix} ∣ ∣ ∣

$1 | \begin{array}{c} 5 & 6 \\ 8 & 9 \end{array} | - 2 | \begin{array}{c} 4 & 6 \\ 7 & 9 \end{array} | + 3 | \begin{array}{c} 4 & 5 \\ 7 & 8 \end{array} |$

1 ∣ ∣ ∣ \begin{matrix} 5 & 6 8 & 9 \end{matrix} ∣ ∣ ∣ - 2 ∣ ∣ ∣ \begin{matrix} 4 & 6 7 & 9 \end{matrix} ∣ ∣ ∣ + 3 ∣ ∣ ∣ \begin{matrix} 4 & 5 7 & 8 \end{matrix} ∣ ∣ ∣

$1 | \begin{array}{c} 5 & 6 \\ 8 & 9 \end{array} | - 2 | \begin{array}{c} 4 & 6 \\ 7 & 9 \end{array} | + 3 | \begin{array}{c} 4 & 5 \\ 7 & 8 \end{array} |$

Step 2

Evaluate

∣ ∣ ∣ \begin{matrix} 5 & 6 8 & 9 \end{matrix} ∣ ∣ ∣

$| \begin{array}{c} 5 & 6 \\ 8 & 9 \end{array} |$ .

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Step 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

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

1 (5 \cdot 9 - 8 \cdot 6) - 2 ∣ ∣ ∣ \begin{matrix} 4 & 6 7 & 9 \end{matrix} ∣ ∣ ∣ + 3 ∣ ∣ ∣ \begin{matrix} 4 & 5 7 & 8 \end{matrix} ∣ ∣ ∣

$1 (5 \cdot 9 - 8 \cdot 6) - 2 | \begin{array}{c} 4 & 6 \\ 7 & 9 \end{array} | + 3 | \begin{array}{c} 4 & 5 \\ 7 & 8 \end{array} |$

Step 2.2

Simplify the determinant.

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Step 2.2.1

Simplify each term.

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Step 2.2.1.1

Multiply

5

$5$ by

9

$9$ .

1 (45 - 8 \cdot 6) - 2 ∣ ∣ ∣ \begin{matrix} 4 & 6 7 & 9 \end{matrix} ∣ ∣ ∣ + 3 ∣ ∣ ∣ \begin{matrix} 4 & 5 7 & 8 \end{matrix} ∣ ∣ ∣

$1 (45 - 8 \cdot 6) - 2 | \begin{array}{c} 4 & 6 \\ 7 & 9 \end{array} | + 3 | \begin{array}{c} 4 & 5 \\ 7 & 8 \end{array} |$

Step 2.2.1.2

Multiply

- 8

$- 8$ by

6

$6$ .

1 (45 - 48) - 2 ∣ ∣ ∣ \begin{matrix} 4 & 6 7 & 9 \end{matrix} ∣ ∣ ∣ + 3 ∣ ∣ ∣ \begin{matrix} 4 & 5 7 & 8 \end{matrix} ∣ ∣ ∣

$1 (45 - 48) - 2 | \begin{array}{c} 4 & 6 \\ 7 & 9 \end{array} | + 3 | \begin{array}{c} 4 & 5 \\ 7 & 8 \end{array} |$

1 (45 - 48) - 2 ∣ ∣ ∣ \begin{matrix} 4 & 6 7 & 9 \end{matrix} ∣ ∣ ∣ + 3 ∣ ∣ ∣ \begin{matrix} 4 & 5 7 & 8 \end{matrix} ∣ ∣ ∣

$1 (45 - 48) - 2 | \begin{array}{c} 4 & 6 \\ 7 & 9 \end{array} | + 3 | \begin{array}{c} 4 & 5 \\ 7 & 8 \end{array} |$

Step 2.2.2

Subtract

48

$48$ from

45

$45$ .

1 \cdot - 3 - 2 ∣ ∣ ∣ \begin{matrix} 4 & 6 7 & 9 \end{matrix} ∣ ∣ ∣ + 3 ∣ ∣ ∣ \begin{matrix} 4 & 5 7 & 8 \end{matrix} ∣ ∣ ∣

$1 \cdot - 3 - 2 | \begin{array}{c} 4 & 6 \\ 7 & 9 \end{array} | + 3 | \begin{array}{c} 4 & 5 \\ 7 & 8 \end{array} |$

1 \cdot - 3 - 2 ∣ ∣ ∣ \begin{matrix} 4 & 6 7 & 9 \end{matrix} ∣ ∣ ∣ + 3 ∣ ∣ ∣ \begin{matrix} 4 & 5 7 & 8 \end{matrix} ∣ ∣ ∣

$1 \cdot - 3 - 2 | \begin{array}{c} 4 & 6 \\ 7 & 9 \end{array} | + 3 | \begin{array}{c} 4 & 5 \\ 7 & 8 \end{array} |$

1 \cdot - 3 - 2 ∣ ∣ ∣ \begin{matrix} 4 & 6 7 & 9 \end{matrix} ∣ ∣ ∣ + 3 ∣ ∣ ∣ \begin{matrix} 4 & 5 7 & 8 \end{matrix} ∣ ∣ ∣

$1 \cdot - 3 - 2 | \begin{array}{c} 4 & 6 \\ 7 & 9 \end{array} | + 3 | \begin{array}{c} 4 & 5 \\ 7 & 8 \end{array} |$

Step 3

Evaluate

∣ ∣ ∣ \begin{matrix} 4 & 6 7 & 9 \end{matrix} ∣ ∣ ∣

$| \begin{array}{c} 4 & 6 \\ 7 & 9 \end{array} |$ .

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Step 3.1

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

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

1 \cdot - 3 - 2 (4 \cdot 9 - 7 \cdot 6) + 3 ∣ ∣ ∣ \begin{matrix} 4 & 5 7 & 8 \end{matrix} ∣ ∣ ∣

$1 \cdot - 3 - 2 (4 \cdot 9 - 7 \cdot 6) + 3 | \begin{array}{c} 4 & 5 \\ 7 & 8 \end{array} |$

Step 3.2

Simplify the determinant.

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Step 3.2.1

Simplify each term.

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Step 3.2.1.1

Multiply

4

$4$ by

9

$9$ .

1 \cdot - 3 - 2 (36 - 7 \cdot 6) + 3 ∣ ∣ ∣ \begin{matrix} 4 & 5 7 & 8 \end{matrix} ∣ ∣ ∣

$1 \cdot - 3 - 2 (36 - 7 \cdot 6) + 3 | \begin{array}{c} 4 & 5 \\ 7 & 8 \end{array} |$

Step 3.2.1.2

Multiply

- 7

$- 7$ by

6

$6$ .

1 \cdot - 3 - 2 (36 - 42) + 3 ∣ ∣ ∣ \begin{matrix} 4 & 5 7 & 8 \end{matrix} ∣ ∣ ∣

$1 \cdot - 3 - 2 (36 - 42) + 3 | \begin{array}{c} 4 & 5 \\ 7 & 8 \end{array} |$

1 \cdot - 3 - 2 (36 - 42) + 3 ∣ ∣ ∣ \begin{matrix} 4 & 5 7 & 8 \end{matrix} ∣ ∣ ∣

$1 \cdot - 3 - 2 (36 - 42) + 3 | \begin{array}{c} 4 & 5 \\ 7 & 8 \end{array} |$

Step 3.2.2

Subtract

42

$42$ from

36

$36$ .

1 \cdot - 3 - 2 \cdot - 6 + 3 ∣ ∣ ∣ \begin{matrix} 4 & 5 7 & 8 \end{matrix} ∣ ∣ ∣

$1 \cdot - 3 - 2 \cdot - 6 + 3 | \begin{array}{c} 4 & 5 \\ 7 & 8 \end{array} |$

1 \cdot - 3 - 2 \cdot - 6 + 3 ∣ ∣ ∣ \begin{matrix} 4 & 5 7 & 8 \end{matrix} ∣ ∣ ∣

$1 \cdot - 3 - 2 \cdot - 6 + 3 | \begin{array}{c} 4 & 5 \\ 7 & 8 \end{array} |$

1 \cdot - 3 - 2 \cdot - 6 + 3 ∣ ∣ ∣ \begin{matrix} 4 & 5 7 & 8 \end{matrix} ∣ ∣ ∣

$1 \cdot - 3 - 2 \cdot - 6 + 3 | \begin{array}{c} 4 & 5 \\ 7 & 8 \end{array} |$

Step 4

Evaluate

∣ ∣ ∣ \begin{matrix} 4 & 5 7 & 8 \end{matrix} ∣ ∣ ∣

$| \begin{array}{c} 4 & 5 \\ 7 & 8 \end{array} |$ .

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Step 4.1

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

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

1 \cdot - 3 - 2 \cdot - 6 + 3 (4 \cdot 8 - 7 \cdot 5)

$1 \cdot - 3 - 2 \cdot - 6 + 3 (4 \cdot 8 - 7 \cdot 5)$

Step 4.2

Simplify the determinant.

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Step 4.2.1

Simplify each term.

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Step 4.2.1.1

Multiply

4

$4$ by

8

$8$ .

1 \cdot - 3 - 2 \cdot - 6 + 3 (32 - 7 \cdot 5)

$1 \cdot - 3 - 2 \cdot - 6 + 3 (32 - 7 \cdot 5)$

Step 4.2.1.2

Multiply

- 7

$- 7$ by

5

$5$ .

1 \cdot - 3 - 2 \cdot - 6 + 3 (32 - 35)

$1 \cdot - 3 - 2 \cdot - 6 + 3 (32 - 35)$

1 \cdot - 3 - 2 \cdot - 6 + 3 (32 - 35)

$1 \cdot - 3 - 2 \cdot - 6 + 3 (32 - 35)$

Step 4.2.2

Subtract

35

$35$ from

32

$32$ .

1 \cdot - 3 - 2 \cdot - 6 + 3 \cdot - 3

$1 \cdot - 3 - 2 \cdot - 6 + 3 \cdot - 3$

1 \cdot - 3 - 2 \cdot - 6 + 3 \cdot - 3

$1 \cdot - 3 - 2 \cdot - 6 + 3 \cdot - 3$

1 \cdot - 3 - 2 \cdot - 6 + 3 \cdot - 3

$1 \cdot - 3 - 2 \cdot - 6 + 3 \cdot - 3$

Step 5

Simplify the determinant.

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Step 5.1

Simplify each term.

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Step 5.1.1

Multiply

- 3

$- 3$ by

1

$1$ .

- 3 - 2 \cdot - 6 + 3 \cdot - 3

$- 3 - 2 \cdot - 6 + 3 \cdot - 3$

Step 5.1.2

Multiply

- 2

$- 2$ by

- 6

$- 6$ .

- 3 + 12 + 3 \cdot - 3

$- 3 + 12 + 3 \cdot - 3$

Step 5.1.3

Multiply

3

$3$ by

- 3

$- 3$ .

- 3 + 12 - 9

$- 3 + 12 - 9$

- 3 + 12 - 9

$- 3 + 12 - 9$

Step 5.2

Add

- 3

$- 3$ and

12

$12$ .

9 - 9

$9 - 9$

Step 5.3

Subtract

9

$9$ from

9

$9$ .

0

$0$

0

$0$