Linear Algebra Examples

$[\begin{array}{c} 5 & 1 & 2 \\ 0 & 1 & 0 \\ 2 & 1 & 4 \end{array}]$

Step 1

Set up the formula to find the characteristic equation $p (λ)$ .

$p (λ) = determinant (A - λ I_{3})$

Step 2

The identity matrix or unit matrix of size $3$ is the $3 \times 3$ square matrix with ones on the main diagonal and zeros elsewhere.

$[\begin{array}{c} 1 & 0 & 0 \\ 0 & 1 & 0 \\ 0 & 0 & 1 \end{array}]$

Step 3

Substitute the known values into $p (λ) = determinant (A - λ I_{3})$ .

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Step 3.1

Substitute $[\begin{array}{c} 5 & 1 & 2 \\ 0 & 1 & 0 \\ 2 & 1 & 4 \end{array}]$ for $A$ .

$p (λ) = determinant ([\begin{array}{c} 5 & 1 & 2 \\ 0 & 1 & 0 \\ 2 & 1 & 4 \end{array}] - λ I_{3})$

Step 3.2

Substitute $[\begin{array}{c} 1 & 0 & 0 \\ 0 & 1 & 0 \\ 0 & 0 & 1 \end{array}]$ for $I_{3}$ .

$p (λ) = determinant ([\begin{array}{c} 5 & 1 & 2 \\ 0 & 1 & 0 \\ 2 & 1 & 4 \end{array}] - λ [\begin{array}{c} 1 & 0 & 0 \\ 0 & 1 & 0 \\ 0 & 0 & 1 \end{array}])$

Step 4

Simplify.

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Step 4.1

Simplify each term.

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Step 4.1.1

Multiply $- λ$ by each element of the matrix.

$p (λ) = determinant ([\begin{array}{c} 5 & 1 & 2 \\ 0 & 1 & 0 \\ 2 & 1 & 4 \end{array}] + [\begin{array}{c} - λ \cdot 1 & - λ \cdot 0 & - λ \cdot 0 \\ - λ \cdot 0 & - λ \cdot 1 & - λ \cdot 0 \\ - λ \cdot 0 & - λ \cdot 0 & - λ \cdot 1 \end{array}])$

Step 4.1.2

Simplify each element in the matrix.

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Step 4.1.2.1

Multiply $- 1$ by $1$ .

$p (λ) = determinant ([\begin{array}{c} 5 & 1 & 2 \\ 0 & 1 & 0 \\ 2 & 1 & 4 \end{array}] + [\begin{array}{c} - λ & - λ \cdot 0 & - λ \cdot 0 \\ - λ \cdot 0 & - λ \cdot 1 & - λ \cdot 0 \\ - λ \cdot 0 & - λ \cdot 0 & - λ \cdot 1 \end{array}])$

Step 4.1.2.2

Multiply $- λ \cdot 0$ .

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Step 4.1.2.2.1

Multiply $0$ by $- 1$ .

$p (λ) = determinant ([\begin{array}{c} 5 & 1 & 2 \\ 0 & 1 & 0 \\ 2 & 1 & 4 \end{array}] + [\begin{array}{c} - λ & 0 λ & - λ \cdot 0 \\ - λ \cdot 0 & - λ \cdot 1 & - λ \cdot 0 \\ - λ \cdot 0 & - λ \cdot 0 & - λ \cdot 1 \end{array}])$

Step 4.1.2.2.2

Multiply $0$ by $λ$ .

$p (λ) = determinant ([\begin{array}{c} 5 & 1 & 2 \\ 0 & 1 & 0 \\ 2 & 1 & 4 \end{array}] + [\begin{array}{c} - λ & 0 & - λ \cdot 0 \\ - λ \cdot 0 & - λ \cdot 1 & - λ \cdot 0 \\ - λ \cdot 0 & - λ \cdot 0 & - λ \cdot 1 \end{array}])$

Step 4.1.2.3

Multiply $- λ \cdot 0$ .

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Step 4.1.2.3.1

Multiply $0$ by $- 1$ .

$p (λ) = determinant ([\begin{array}{c} 5 & 1 & 2 \\ 0 & 1 & 0 \\ 2 & 1 & 4 \end{array}] + [\begin{array}{c} - λ & 0 & 0 λ \\ - λ \cdot 0 & - λ \cdot 1 & - λ \cdot 0 \\ - λ \cdot 0 & - λ \cdot 0 & - λ \cdot 1 \end{array}])$

Step 4.1.2.3.2

Multiply $0$ by $λ$ .

$p (λ) = determinant ([\begin{array}{c} 5 & 1 & 2 \\ 0 & 1 & 0 \\ 2 & 1 & 4 \end{array}] + [\begin{array}{c} - λ & 0 & 0 \\ - λ \cdot 0 & - λ \cdot 1 & - λ \cdot 0 \\ - λ \cdot 0 & - λ \cdot 0 & - λ \cdot 1 \end{array}])$

Step 4.1.2.4

Multiply $- λ \cdot 0$ .

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Step 4.1.2.4.1

Multiply $0$ by $- 1$ .

$p (λ) = determinant ([\begin{array}{c} 5 & 1 & 2 \\ 0 & 1 & 0 \\ 2 & 1 & 4 \end{array}] + [\begin{array}{c} - λ & 0 & 0 \\ 0 λ & - λ \cdot 1 & - λ \cdot 0 \\ - λ \cdot 0 & - λ \cdot 0 & - λ \cdot 1 \end{array}])$

Step 4.1.2.4.2

Multiply $0$ by $λ$ .

$p (λ) = determinant ([\begin{array}{c} 5 & 1 & 2 \\ 0 & 1 & 0 \\ 2 & 1 & 4 \end{array}] + [\begin{array}{c} - λ & 0 & 0 \\ 0 & - λ \cdot 1 & - λ \cdot 0 \\ - λ \cdot 0 & - λ \cdot 0 & - λ \cdot 1 \end{array}])$

Step 4.1.2.5

Multiply $- 1$ by $1$ .

$p (λ) = determinant ([\begin{array}{c} 5 & 1 & 2 \\ 0 & 1 & 0 \\ 2 & 1 & 4 \end{array}] + [\begin{array}{c} - λ & 0 & 0 \\ 0 & - λ & - λ \cdot 0 \\ - λ \cdot 0 & - λ \cdot 0 & - λ \cdot 1 \end{array}])$

Step 4.1.2.6

Multiply $- λ \cdot 0$ .

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Step 4.1.2.6.1

Multiply $0$ by $- 1$ .

$p (λ) = determinant ([\begin{array}{c} 5 & 1 & 2 \\ 0 & 1 & 0 \\ 2 & 1 & 4 \end{array}] + [\begin{array}{c} - λ & 0 & 0 \\ 0 & - λ & 0 λ \\ - λ \cdot 0 & - λ \cdot 0 & - λ \cdot 1 \end{array}])$

Step 4.1.2.6.2

Multiply $0$ by $λ$ .

$p (λ) = determinant ([\begin{array}{c} 5 & 1 & 2 \\ 0 & 1 & 0 \\ 2 & 1 & 4 \end{array}] + [\begin{array}{c} - λ & 0 & 0 \\ 0 & - λ & 0 \\ - λ \cdot 0 & - λ \cdot 0 & - λ \cdot 1 \end{array}])$

Step 4.1.2.7

Multiply $- λ \cdot 0$ .

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Step 4.1.2.7.1

Multiply $0$ by $- 1$ .

$p (λ) = determinant ([\begin{array}{c} 5 & 1 & 2 \\ 0 & 1 & 0 \\ 2 & 1 & 4 \end{array}] + [\begin{array}{c} - λ & 0 & 0 \\ 0 & - λ & 0 \\ 0 λ & - λ \cdot 0 & - λ \cdot 1 \end{array}])$

Step 4.1.2.7.2

Multiply $0$ by $λ$ .

$p (λ) = determinant ([\begin{array}{c} 5 & 1 & 2 \\ 0 & 1 & 0 \\ 2 & 1 & 4 \end{array}] + [\begin{array}{c} - λ & 0 & 0 \\ 0 & - λ & 0 \\ 0 & - λ \cdot 0 & - λ \cdot 1 \end{array}])$

Step 4.1.2.8

Multiply $- λ \cdot 0$ .

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Step 4.1.2.8.1

Multiply $0$ by $- 1$ .

$p (λ) = determinant ([\begin{array}{c} 5 & 1 & 2 \\ 0 & 1 & 0 \\ 2 & 1 & 4 \end{array}] + [\begin{array}{c} - λ & 0 & 0 \\ 0 & - λ & 0 \\ 0 & 0 λ & - λ \cdot 1 \end{array}])$

Step 4.1.2.8.2

Multiply $0$ by $λ$ .

$p (λ) = determinant ([\begin{array}{c} 5 & 1 & 2 \\ 0 & 1 & 0 \\ 2 & 1 & 4 \end{array}] + [\begin{array}{c} - λ & 0 & 0 \\ 0 & - λ & 0 \\ 0 & 0 & - λ \cdot 1 \end{array}])$

Step 4.1.2.9

Multiply $- 1$ by $1$ .

$p (λ) = determinant ([\begin{array}{c} 5 & 1 & 2 \\ 0 & 1 & 0 \\ 2 & 1 & 4 \end{array}] + [\begin{array}{c} - λ & 0 & 0 \\ 0 & - λ & 0 \\ 0 & 0 & - λ \end{array}])$

Step 4.2

Add the corresponding elements.

$p (λ) = determinant [\begin{array}{c} 5 - λ & 1 + 0 & 2 + 0 \\ 0 + 0 & 1 - λ & 0 + 0 \\ 2 + 0 & 1 + 0 & 4 - λ \end{array}]$

Step 4.3

Simplify each element.

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Step 4.3.1

Add $1$ and $0$ .

$p (λ) = determinant [\begin{array}{c} 5 - λ & 1 & 2 + 0 \\ 0 + 0 & 1 - λ & 0 + 0 \\ 2 + 0 & 1 + 0 & 4 - λ \end{array}]$

Step 4.3.2

Add $2$ and $0$ .

$p (λ) = determinant [\begin{array}{c} 5 - λ & 1 & 2 \\ 0 + 0 & 1 - λ & 0 + 0 \\ 2 + 0 & 1 + 0 & 4 - λ \end{array}]$

Step 4.3.3

Add $0$ and $0$ .

$p (λ) = determinant [\begin{array}{c} 5 - λ & 1 & 2 \\ 0 & 1 - λ & 0 + 0 \\ 2 + 0 & 1 + 0 & 4 - λ \end{array}]$

Step 4.3.4

Add $0$ and $0$ .

$p (λ) = determinant [\begin{array}{c} 5 - λ & 1 & 2 \\ 0 & 1 - λ & 0 \\ 2 + 0 & 1 + 0 & 4 - λ \end{array}]$

Step 4.3.5

Add $2$ and $0$ .

$p (λ) = determinant [\begin{array}{c} 5 - λ & 1 & 2 \\ 0 & 1 - λ & 0 \\ 2 & 1 + 0 & 4 - λ \end{array}]$

Step 4.3.6

Add $1$ and $0$ .

$p (λ) = determinant [\begin{array}{c} 5 - λ & 1 & 2 \\ 0 & 1 - λ & 0 \\ 2 & 1 & 4 - λ \end{array}]$

Step 5

Find the determinant.

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Step 5.1

Choose the row or column with the most $0$ elements. If there are no $0$ elements choose any row or column. Multiply every element in row $2$ by its cofactor and add.

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Step 5.1.1

Consider the corresponding sign chart.

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

Step 5.1.2

The cofactor is the minor with the sign changed if the indices match a $-$ position on the sign chart.

Step 5.1.3

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

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

Step 5.1.4

Multiply element $a_{21}$ by its cofactor.

$0 | \begin{array}{c} 1 & 2 \\ 1 & 4 - λ \end{array} |$

Step 5.1.5

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

$| \begin{array}{c} 5 - λ & 2 \\ 2 & 4 - λ \end{array} |$

Step 5.1.6

Multiply element $a_{22}$ by its cofactor.

$(1 - λ) | \begin{array}{c} 5 - λ & 2 \\ 2 & 4 - λ \end{array} |$

Step 5.1.7

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

$| \begin{array}{c} 5 - λ & 1 \\ 2 & 1 \end{array} |$

Step 5.1.8

Multiply element $a_{23}$ by its cofactor.

$0 | \begin{array}{c} 5 - λ & 1 \\ 2 & 1 \end{array} |$

Step 5.1.9

Add the terms together.

$p (λ) = 0 | \begin{array}{c} 1 & 2 \\ 1 & 4 - λ \end{array} | + (1 - λ) | \begin{array}{c} 5 - λ & 2 \\ 2 & 4 - λ \end{array} | + 0 | \begin{array}{c} 5 - λ & 1 \\ 2 & 1 \end{array} |$

Step 5.2

Multiply $0$ by $| \begin{array}{c} 1 & 2 \\ 1 & 4 - λ \end{array} |$ .

$p (λ) = 0 + (1 - λ) | \begin{array}{c} 5 - λ & 2 \\ 2 & 4 - λ \end{array} | + 0 | \begin{array}{c} 5 - λ & 1 \\ 2 & 1 \end{array} |$

Step 5.3

Multiply $0$ by $| \begin{array}{c} 5 - λ & 1 \\ 2 & 1 \end{array} |$ .

$p (λ) = 0 + (1 - λ) | \begin{array}{c} 5 - λ & 2 \\ 2 & 4 - λ \end{array} | + 0$

Step 5.4

Evaluate $| \begin{array}{c} 5 - λ & 2 \\ 2 & 4 - λ \end{array} |$ .

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Step 5.4.1

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

$p (λ) = 0 + (1 - λ) ((5 - λ) (4 - λ) - 2 \cdot 2) + 0$

Step 5.4.2

Simplify the determinant.

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Step 5.4.2.1

Simplify each term.

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Step 5.4.2.1.1

Expand $(5 - λ) (4 - λ)$ using the FOIL Method.

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Step 5.4.2.1.1.1

Apply the distributive property.

$p (λ) = 0 + (1 - λ) (5 (4 - λ) - λ (4 - λ) - 2 \cdot 2) + 0$

Step 5.4.2.1.1.2

Apply the distributive property.

$p (λ) = 0 + (1 - λ) (5 \cdot 4 + 5 (- λ) - λ (4 - λ) - 2 \cdot 2) + 0$

Step 5.4.2.1.1.3

Apply the distributive property.

$p (λ) = 0 + (1 - λ) (5 \cdot 4 + 5 (- λ) - λ \cdot 4 - λ (- λ) - 2 \cdot 2) + 0$

Step 5.4.2.1.2

Simplify and combine like terms.

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Step 5.4.2.1.2.1

Simplify each term.

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Step 5.4.2.1.2.1.1

Multiply $5$ by $4$ .

$p (λ) = 0 + (1 - λ) (20 + 5 (- λ) - λ \cdot 4 - λ (- λ) - 2 \cdot 2) + 0$

Step 5.4.2.1.2.1.2

Multiply $- 1$ by $5$ .

$p (λ) = 0 + (1 - λ) (20 - 5 λ - λ \cdot 4 - λ (- λ) - 2 \cdot 2) + 0$

Step 5.4.2.1.2.1.3

Multiply $4$ by $- 1$ .

$p (λ) = 0 + (1 - λ) (20 - 5 λ - 4 λ - λ (- λ) - 2 \cdot 2) + 0$

Step 5.4.2.1.2.1.4

Rewrite using the commutative property of multiplication.

$p (λ) = 0 + (1 - λ) (20 - 5 λ - 4 λ - 1 \cdot - 1 λ \cdot λ - 2 \cdot 2) + 0$

Step 5.4.2.1.2.1.5

Multiply $λ$ by $λ$ by adding the exponents.

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Step 5.4.2.1.2.1.5.1

Move $λ$ .

$p (λ) = 0 + (1 - λ) (20 - 5 λ - 4 λ - 1 \cdot - 1 (λ \cdot λ) - 2 \cdot 2) + 0$

Step 5.4.2.1.2.1.5.2

Multiply $λ$ by $λ$ .

$p (λ) = 0 + (1 - λ) (20 - 5 λ - 4 λ - 1 \cdot - 1 λ^{2} - 2 \cdot 2) + 0$

Step 5.4.2.1.2.1.6

Multiply $- 1$ by $- 1$ .

$p (λ) = 0 + (1 - λ) (20 - 5 λ - 4 λ + 1 λ^{2} - 2 \cdot 2) + 0$

Step 5.4.2.1.2.1.7

Multiply $λ^{2}$ by $1$ .

$p (λ) = 0 + (1 - λ) (20 - 5 λ - 4 λ + λ^{2} - 2 \cdot 2) + 0$

Step 5.4.2.1.2.2

Subtract $4 λ$ from $- 5 λ$ .

$p (λ) = 0 + (1 - λ) (20 - 9 λ + λ^{2} - 2 \cdot 2) + 0$

Step 5.4.2.1.3

Multiply $- 2$ by $2$ .

$p (λ) = 0 + (1 - λ) (20 - 9 λ + λ^{2} - 4) + 0$

Step 5.4.2.2

Subtract $4$ from $20$ .

$p (λ) = 0 + (1 - λ) (- 9 λ + λ^{2} + 16) + 0$

Step 5.4.2.3

Reorder $- 9 λ$ and $λ^{2}$ .

$p (λ) = 0 + (1 - λ) (λ^{2} - 9 λ + 16) + 0$

Step 5.5

Simplify the determinant.

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Step 5.5.1

Combine the opposite terms in $0 + (1 - λ) (λ^{2} - 9 λ + 16) + 0$ .

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Step 5.5.1.1

Add $0$ and $(1 - λ) (λ^{2} - 9 λ + 16)$ .

$p (λ) = (1 - λ) (λ^{2} - 9 λ + 16) + 0$

Step 5.5.1.2

Add $(1 - λ) (λ^{2} - 9 λ + 16)$ and $0$ .

$p (λ) = (1 - λ) (λ^{2} - 9 λ + 16)$

Step 5.5.2

Expand $(1 - λ) (λ^{2} - 9 λ + 16)$ by multiplying each term in the first expression by each term in the second expression.

$p (λ) = 1 λ^{2} + 1 (- 9 λ) + 1 \cdot 16 - λ \cdot λ^{2} - λ (- 9 λ) - λ \cdot 16$

Step 5.5.3

Simplify each term.

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Step 5.5.3.1

Multiply $λ^{2}$ by $1$ .

$p (λ) = λ^{2} + 1 (- 9 λ) + 1 \cdot 16 - λ \cdot λ^{2} - λ (- 9 λ) - λ \cdot 16$

Step 5.5.3.2

Multiply $- 9 λ$ by $1$ .

$p (λ) = λ^{2} - 9 λ + 1 \cdot 16 - λ \cdot λ^{2} - λ (- 9 λ) - λ \cdot 16$

Step 5.5.3.3

Multiply $16$ by $1$ .

$p (λ) = λ^{2} - 9 λ + 16 - λ \cdot λ^{2} - λ (- 9 λ) - λ \cdot 16$

Step 5.5.3.4

Multiply $λ$ by $λ^{2}$ by adding the exponents.

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Step 5.5.3.4.1

Move $λ^{2}$ .

$p (λ) = λ^{2} - 9 λ + 16 - (λ^{2} λ) - λ (- 9 λ) - λ \cdot 16$

Step 5.5.3.4.2

Multiply $λ^{2}$ by $λ$ .

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Step 5.5.3.4.2.1

Raise $λ$ to the power of $1$ .

$p (λ) = λ^{2} - 9 λ + 16 - (λ^{2} λ^{1}) - λ (- 9 λ) - λ \cdot 16$

Step 5.5.3.4.2.2

Use the power rule $a^{m} a^{n} = a^{m + n}$ to combine exponents.

$p (λ) = λ^{2} - 9 λ + 16 - λ^{2 + 1} - λ (- 9 λ) - λ \cdot 16$

Step 5.5.3.4.3

Add $2$ and $1$ .

$p (λ) = λ^{2} - 9 λ + 16 - λ^{3} - λ (- 9 λ) - λ \cdot 16$

Step 5.5.3.5

Rewrite using the commutative property of multiplication.

$p (λ) = λ^{2} - 9 λ + 16 - λ^{3} - 1 \cdot - 9 λ \cdot λ - λ \cdot 16$

Step 5.5.3.6

Multiply $λ$ by $λ$ by adding the exponents.

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Step 5.5.3.6.1

Move $λ$ .

$p (λ) = λ^{2} - 9 λ + 16 - λ^{3} - 1 \cdot - 9 (λ \cdot λ) - λ \cdot 16$

Step 5.5.3.6.2

Multiply $λ$ by $λ$ .

$p (λ) = λ^{2} - 9 λ + 16 - λ^{3} - 1 \cdot - 9 λ^{2} - λ \cdot 16$

Step 5.5.3.7

Multiply $- 1$ by $- 9$ .

$p (λ) = λ^{2} - 9 λ + 16 - λ^{3} + 9 λ^{2} - λ \cdot 16$

Step 5.5.3.8

Multiply $16$ by $- 1$ .

$p (λ) = λ^{2} - 9 λ + 16 - λ^{3} + 9 λ^{2} - 16 λ$

Step 5.5.4

Add $λ^{2}$ and $9 λ^{2}$ .

$p (λ) = 10 λ^{2} - 9 λ + 16 - λ^{3} - 16 λ$

Step 5.5.5

Subtract $16 λ$ from $- 9 λ$ .

$p (λ) = 10 λ^{2} - 25 λ + 16 - λ^{3}$

Step 5.5.6

Move $16$ .

$p (λ) = 10 λ^{2} - 25 λ - λ^{3} + 16$

Step 5.5.7

Move $- 25 λ$ .

$p (λ) = 10 λ^{2} - λ^{3} - 25 λ + 16$

Step 5.5.8

Reorder $10 λ^{2}$ and $- λ^{3}$ .

$p (λ) = - λ^{3} + 10 λ^{2} - 25 λ + 16$