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Determinants · Determinants

Calculating Determinants

Before you start, make sure you know: Gaussian elimination (High School Year 2 (semesters 3 and 4))

The corner stationery store keeps its inventory in a $4\times4$ matrix (four products, four branches). Sarrus only works for order 3, so how do you find the determinant of that matrix without getting lost in 24 terms? Here you'll see how to take advantage of zeros, break the determinant into smaller pieces, and simplify with properties before calculating.

Review: order 2 and Sarrus's rule

For a $2\times2$ matrix, the determinant is the product of the main diagonal minus the product of the secondary diagonal:

$$\begin{vmatrix} a & b \\ c & d \end{vmatrix} = ad - bc$$

For order 3 you use Sarrus's rule: add the three products in the direction of the main diagonal and subtract the three products in the direction of the secondary diagonal.

Determinants also measure areas. The area of the triangle with vertices $(0, 0)$, $(x_1, y_1)$ and $(x_2, y_2)$ is

$$\text{Área} = \tfrac{1}{2}\left|\begin{vmatrix} x_1 & y_1 \\ x_2 & y_2 \end{vmatrix}\right|$$

For $(0, 0)$, $(3, 1)$ and $(2, 4)$: $\begin{vmatrix} 3 & 1 \\ 2 & 4 \end{vmatrix} = 12 - 2 = 10$, so the area is $5$. The absolute value is used because an area is not negative.

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Un joven streamer* está analizando el crecimiento de su canal. Para comparar el impacto de dos de sus vídeos, organiza los datos de "Likes" y "Compartidos" en una matriz…

Un joven streamer está analizando el crecimiento de su canal. Para comparar el impacto de dos de sus vídeos, organiza los datos de "Likes" y "Compartidos" en una matriz $A$ de orden $2 \times 2$: cada fila corresponde a un vídeo y cada columna a un tipo de dato. Calcula el determinante de la matriz $A$ para obtener un índice de repercusión:

$$A = \begin{pmatrix} 120 & 45 \\ 30 & 10 \end{pmatrix}$$ Data matrix with rows Video 1 and Video 2 and columns Likes and Shares, written between determinant bars Likes Shares Video 1 Video 2 120 45 30 10
Solve
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Calculate the value of the determinant of the matrix $B$ using Sarrus' rule. This matrix represents the inventory of components (chips, screens and batteries) of three…

Calculate the value of the determinant of the matrix $B$ using Sarrus' rule. This matrix represents the inventory of components (chips, screens and batteries) of three warehouses of a tech store:

$$B = \begin{pmatrix} 1 & 0 & 3 \\ 2 & 1 & 1 \\ 0 & 2 & 2 \end{pmatrix}$$
Solve
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In a graphic design program, the area of a triangle with one vertex at the origin $(0,0)$ and the other two at $(x_1, y_1)$ and $(x_2, y_2)$ can be calculated as half…

In a graphic design program, the area of a triangle with one vertex at the origin $(0,0)$ and the other two at $(x_1, y_1)$ and $(x_2, y_2)$ can be calculated as half the absolute value of the determinant formed by their coordinates. Calculate the area of the triangle formed by the points $(0,0)$, $(4, 2)$ and $(1, 5)$.

Triangle with vertices at the origin, at the point (4,2) and at the point (1,5), drawn on Cartesian axes x y 4 2 1 5 (0,0) (4,2) (1,5)
Solve

Equations with determinants

If an unknown appears in a determinant, expanding it gives you an ordinary equation. You calculate the determinant, set it equal to a value, and solve.

This is useful for finding out when a matrix has no inverse: a square matrix is invertible if and only if $\det A \neq 0$. For $\begin{pmatrix} k & 2 \\ 8 & k \end{pmatrix}$ you get $k^2 - 16 = 0$, so it is not invertible when $k = 4$ or $k = -4$.

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In a strategy video game, the power of a magic attack is calculated by solving an equation based on a determinant. Find the value of $x$ so that the following…

In a strategy video game, the power of a magic attack is calculated by solving an equation based on a determinant. Find the value of $x$ so that the following determinant is equal to $0$:

$$\begin{vmatrix} x & 8 \\ 2 & x \end{vmatrix} = 0$$
Solve
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Find all real values of $x$ for which the following equality between determinants is satisfied: $$\begin{vmatrix} x & 1 & 1 \\ 1 & x & 1 \\ 1 & 1 & x \end{vmatrix} =…

Find all real values of $x$ for which the following equality between determinants is satisfied:

$$\begin{vmatrix} x & 1 & 1 \\ 1 & x & 1 \\ 1 & 1 & x \end{vmatrix} = \begin{vmatrix} x & -1 \\ 2 & 1 \end{vmatrix}$$
Solve
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Determine for which values of the real parameter $k$ the matrix $A$ does not have an inverse. To do this, set its determinant to zero and solve the resulting equation:…

Determine for which values of the real parameter $k$ the matrix $A$ does not have an inverse. To do this, set its determinant to zero and solve the resulting equation:

$$A = \begin{pmatrix} k & 1 & 2 \\ 2 & k+1 & 0 \\ 1 & k & 1 \end{pmatrix}$$
Solve

Determinant of a triangular matrix

Upper triangular matrix of order 3 with the main diagonal highlighted and its determinant as the product of the diagonal. 5 −2 1 0 3 4 0 0 2 det = 5 · 3 · 2 = 30

A very common use: eigenvalues. If $A$ is triangular, $A-\lambda I$ is too, because only the diagonal changes. Its determinant is the product of the factors $(a_{ii}-\lambda)$, and it vanishes when $\lambda$ is any entry on the diagonal of $A$.

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A logistics company uses an upper triangular matrix to organize its delivery routes efficiently. Calculate the determinant of the matrix $L$, knowing that for this type…

A logistics company uses an upper triangular matrix to organize its delivery routes efficiently. Calculate the determinant of the matrix $L$, knowing that for this type of matrix the calculation is greatly simplified:

$$L = \begin{pmatrix} 5 & -2 & 1 \\ 0 & 3 & 4 \\ 0 & 0 & 2 \end{pmatrix}$$
Solve
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Solve the matrix equation $|X| = 0$, where $X = A - \lambda I$, $A$ being the given matrix and $I$ the identity matrix of order 3. Find the values of $\lambda$ that make…

Solve the matrix equation $|X| = 0$, where $X = A - \lambda I$, $A$ being the given matrix and $I$ the identity matrix of order 3. Find the values of $\lambda$ that make the determinant zero (eigenvalues):

$$A = \begin{pmatrix} 1 & 0 & -1 \\ 0 & 2 & 3 \\ 0 & 0 & -1 \end{pmatrix}$$
Solve

Minor and cofactor

The signs $(-1)^{i+j}$ form a checkerboard that starts with $+$ in the upper left corner:

$$\begin{pmatrix} + & - & + \\ - & + & - \\ + & - & + \end{pmatrix}$$ Matrix of order 3 with a row and a column shaded to illustrate how the minor of an entry is obtained. a₂₁ a₁₂ a₁₃ a₃₂ a₃₃ cover row 2 and column 1: minor of a₂₁
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Given the matrix $M$, calculate its minor $\alpha_{21}$ and its cofactor $A_{21}$: $$M = \begin{pmatrix} 2 & 1 & 3 \\ 4 & 0 & 5 \\ -1 & 2 & 1 \end{pmatrix}$$

Given the matrix $M$, calculate its minor $\alpha_{21}$ and its cofactor $A_{21}$:

$$M = \begin{pmatrix} 2 & 1 & 3 \\ 4 & 0 & 5 \\ -1 & 2 & 1 \end{pmatrix}$$
Solve

Cofactor expansion (Laplace)

Matrix of order 3 with a highlighted row containing two zeros, which reduces the cofactor expansion to a single product. 203 010 105 row with two zeros: only one product remains
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Calculate the value of the following determinant of order 4 by expanding along the elements of the most suitable row or column (Laplace expansion): $$\Delta =…

Calculate the value of the following determinant of order 4 by expanding along the elements of the most suitable row or column (Laplace expansion):

$$\Delta = \begin{vmatrix} 2 & 0 & 3 & 4 \\ 0 & 1 & -1 & 0 \\ 1 & 0 & 5 & 0 \\ 3 & 2 & 0 & 1 \end{vmatrix}$$
Solve
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A GPS positioning system uses four satellites to triangulate a position in space. The volume of the tetrahedron formed by the four satellites is related to the absolute…

A GPS positioning system uses four satellites to triangulate a position in space. The volume of the tetrahedron formed by the four satellites is related to the absolute value of the determinant of their coordinates matrix. Calculate with reasoning the value of the following determinant of order 4, which represents a specific configuration of these satellites:

$$\Delta = \begin{vmatrix} a+1 & a & a & a \\ a & a+1 & a & a \\ a & a & a+1 & a \\ a & a & a & a+1 \end{vmatrix}$$

Express the result as a polynomial in $a$ simplified as far as possible.

h S₁ S₂ S₃ S₄
Solve

Properties that save computations

These properties let you transform a determinant without expanding it.

A useful case: why is $\begin{vmatrix} 1 & 2 & 3 \\ 0 & 1 & 4 \\ 1 & 3 & 7 \end{vmatrix} = 0$ without calculating anything? Because row 3 is the sum of rows 1 and 2.

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Without performing complex calculations, justify why the following determinants equal $0$ using the properties of determinants: a)…

Without performing complex calculations, justify why the following determinants equal $0$ using the properties of determinants:

  1. a) $D_1 = \begin{vmatrix} 1 & 2 & 3 \\ 0 & 0 & 0 \\ 5 & 1 & 4 \end{vmatrix}$
  1. b) $D_2 = \begin{vmatrix} 2 & -1 & 4 \\ 2 & -1 & 4 \\ 10 & 20 & 30 \end{vmatrix}$
  1. c) $D_3 = \begin{vmatrix} 1 & 3 & 2 \\ 2 & 6 & 4 \\ 0 & 1 & 5 \end{vmatrix}$
Solve
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We know that the determinant of a matrix $A$ of order $2 \times 2$ is $|A| = 5$. If we multiply all the elements of the second row by $3$, what will be the value of the…

We know that the determinant of a matrix $A$ of order $2 \times 2$ is $|A| = 5$. If we multiply all the elements of the second row by $3$, what will be the value of the new determinant? Justify your answer based on the properties.

Solve
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Knowing that the determinant of a square matrix $B$ of order 3 is: $$\begin{vmatrix} a & b & c \\ d & e & f \\ g & h & i \end{vmatrix} = 10$$ Calculate, without…

Knowing that the determinant of a square matrix $B$ of order 3 is:

$$\begin{vmatrix} a & b & c \\ d & e & f \\ g & h & i \end{vmatrix} = 10$$

Calculate, without expanding by Sarrus' rule, the value of the determinant of the matrix $C$:

$$\begin{vmatrix} 3d & 3e & 3f \\ a+g & b+h & c+i \\ g & h & i \end{vmatrix}$$
Solve
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Given the square matrix $A$ of order 3, it is known that its determinant is $|A| = -2$. Use the properties of determinants to calculate with reasoning the value of: a)…

Given the square matrix $A$ of order 3, it is known that its determinant is $|A| = -2$. Use the properties of determinants to calculate with reasoning the value of:

a) $|A^3|$

b) $|3 \times A^t|$

c) $|A^{-1} \times (2A)|$

d) The determinant of the matrix resulting from swapping the first two rows of $A$ and multiplying the third row by $-5$.

|A| = −2 F₁ F₂ F₃ n = 3 Transformation (d) ↕ F₂ F₁ x(−5) −5·F₃ KEY PROPERTIES TO SOLVE (a, b, c) |Aⁿ| = |A|ⁿ |Aᵗ| = |A| |A⁻¹| = 1/|A| |k·A| = kⁿ·|A|
Solve
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Let $C_1, C_2, C_3$ be the three columns of a square matrix $M$ of order 3, such that its determinant is $|M| = -5$. Calculate, justifying each step with the…

Let $C_1, C_2, C_3$ be the three columns of a square matrix $M$ of order 3, such that its determinant is $|M| = -5$. Calculate, justifying each step with the corresponding property, the value of the determinant of the matrix whose columns are:

$$| 3C_1 - C_2, \quad 2C_3, \quad C_2 + C_1 |$$
Solve
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Let $A = (a_{ij})$ be the matrix of order $3 \times 3$ defined by $a_{ij} = i + j$. Consider also a matrix $B$ such that its determinant is $|B| = 2$. Using exclusively…

Let $A = (a_{ij})$ be the matrix of order $3 \times 3$ defined by $a_{ij} = i + j$. Consider also a matrix $B$ such that its determinant is $|B| = 2$. Using exclusively the properties of determinants, calculate with reasoning the value of the determinant of the matrix $M$, where:

$$M = (2A^t \times B^{-1})^2$$

Explain whether the result obtained allows us to state that the matrix $M$ has an inverse.

A = a₁₁ a₁₂ a₁₃ a₂₁ a₂₂ a₂₃ a₃₁ a₃₂ a₃₃ aᵢⱼ = i+j 2 3 4 3 4 5 4 5 6 + (1,1,1) + (1,1,1)
Solve

Gaussian elimination for determinants

Row operation R2 minus 3 R1 that makes an entry below the diagonal zero without changing the determinant. 120 314 012 R₂ − 3R₁ → 120 0−54 012 the determinant does not change
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Calculate the value of the determinant of the matrix $M$ of order 4 by first transforming it into an upper triangular matrix using Gaussian elimination (creating zeros…

Calculate the value of the determinant of the matrix $M$ of order 4 by first transforming it into an upper triangular matrix using Gaussian elimination (creating zeros below the main diagonal):

$$M = \begin{pmatrix} 1 & 2 & 0 & 3 \\ -1 & 0 & 2 & 1 \\ 0 & 1 & -1 & 2 \\ 2 & 3 & 1 & 0 \end{pmatrix}$$ 1 2 0 3 −1 0 2 1 0 1 −1 2 2 3 1 0 M Gauss a₁₁ a₁₂ a₁₃ a₁₄ 0 a₂₂ a₂₃ a₂₄ 0 0 a₃₃ a₃₄ 0 0 0 a₄₄ U |M| = |U| = a₁₁ · a₂₂ · a₃₃ · a₄₄
Solve

Determinants with parameters: factoring tricks

When the determinant has letters, avoid Sarrus at all costs: it gives you a long polynomial that you then don't know how to factor. With properties, you can make the factors appear right away.

Other tricks: if the determinant vanishes for $x=a$ because two rows become equal, then $(x-a)$ is a factor. And in Vandermonde determinants (columns $1, a, a^2$), subtracting columns or rows produces the factors $(b-a)$, $(c-a)$ and $(c-b)$.

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In the development of a strategy video game, the positions of three units on the map are given by the rows of a matrix. The determinant of this matrix indicates the…

In the development of a strategy video game, the positions of three units on the map are given by the rows of a matrix. The determinant of this matrix indicates the battalion's area of influence. Calculate the value of $a$ so that the area of influence is zero (units collinear or linearly dependent) in the following system:

$$\begin{vmatrix} 1 & a & a^2 \\ 1 & 2 & 4 \\ 1 & -3 & 9 \end{vmatrix} = 0$$

Note: This is a Vandermonde determinant. Try to simplify it using column properties before calculating it.

Solve
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In the design of an aeronautical component, structural stability is given by the determinant of a stress matrix $T$. It is known that the determinant of the following…

In the design of an aeronautical component, structural stability is given by the determinant of a stress matrix $T$. It is known that the determinant of the following matrix depends on a load parameter $k$:

$$|T| = \begin{vmatrix} 1 & 1 & 1 \\ k & 2 & 1 \\ k^2 & 4 & 1 \end{vmatrix}$$
  1. a) Calculate the value of the determinant in terms of $k$ using the properties of determinants to simplify the calculation (avoid direct expansion by Sarrus' rule).
  2. b) Determine for which values of $k$ the component is unstable ($|T| = 0$).
  3. c) If $k=3$, what relationship exists between the rows of the matrix?
Solve
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Solve the following cubic equation using the properties of determinants to factorise the resulting polynomial: $$\begin{vmatrix} x & 2 & 3 \\ 2 & x & 3 \\ 2 & 3 & x…

Solve the following cubic equation using the properties of determinants to factorise the resulting polynomial:

$$\begin{vmatrix} x & 2 & 3 \\ 2 & x & 3 \\ 2 & 3 & x \end{vmatrix} = 0$$

State step by step the elementary operations performed on the rows or columns.

Solve
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In a telecommunications network, flow capacity is modelled using the determinant of the adjacency matrix $W$. $W$ is defined as: $$W = \begin{vmatrix} 1 & a & a^2 \\ 1 &…

In a telecommunications network, flow capacity is modelled using the determinant of the adjacency matrix $W$. $W$ is defined as:

$$W = \begin{vmatrix} 1 & a & a^2 \\ 1 & b & b^2 \\ 1 & c & c^2 \end{vmatrix}$$
  1. a) Show that, without expanding the determinant by Sarrus' rule, $W = (b-a)(c-a)(c-b)$.
  2. b) If the nodes $a, b, c$ represent transmission frequencies, what condition must they satisfy for the determinant to be non-zero? Justify the importance of this condition for the independence of the signals.
W = 1 1 1 a b c a² b² c² |W| = (b-a)(c-a)(c-b) a b c b-a c-b c-a
Solve

Inverse, adjugate, and rank with determinants

The determinant tells you when a matrix has an inverse and, with the adjugate, gives you shortcuts.

Rank. The rank of a matrix is the largest order of a nonzero minor. For a $3\times3$ matrix:

  • If $\det A \neq 0$, the rank is $3$.

  • If $\det A = 0$, the rank is $2$ if some $2\times2$ minor is nonzero, and less than $2$ if they are all zero.

With parameters: set the determinant equal to zero to find the special values, and then study each case separately.

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Given the matrix depending on a real parameter $\lambda$: $$A(\lambda) = \begin{pmatrix} \lambda & 1 & 2 \\ 1 & \lambda & 0 \\ \lambda & 1 & \lambda \end{pmatrix}$$ a)…

Given the matrix depending on a real parameter $\lambda$:

$$A(\lambda) = \begin{pmatrix} \lambda & 1 & 2 \\ 1 & \lambda & 0 \\ \lambda & 1 & \lambda \end{pmatrix}$$
  1. a) Find the values of $\lambda$ for which the matrix $A(\lambda)$ does not have an inverse.
  2. b) For $\lambda = 0$, solve the matrix equation $A \times X \times A^t = I$, where $I$ is the identity matrix of order 3. Calculate $|X|$ indirectly before finding the matrix $X$.
Solve
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Determine the rank of the following matrix $A$ according to the values of the real parameters $m$ and $n$: $$A = \begin{pmatrix} 1 & 0 & 2 \\ m & 0 & n \\ 3 & -1 & 4…

Determine the rank of the following matrix $A$ according to the values of the real parameters $m$ and $n$:

$$A = \begin{pmatrix} 1 & 0 & 2 \\ m & 0 & n \\ 3 & -1 & 4 \end{pmatrix}$$

For the case where $m=1$ and $n=2$, calculate the determinant of the adjugate matrix of $A$, that is, $|Adj(A)|$, without calculating the elements of the adjugate matrix beforehand. (Hint: use the relationship between a matrix, its adjugate and its determinant).

Solve

Summary

Go back to the beginning: for the $4\times4$ inventory matrix, don't use Sarrus; look for the row or column with the most zeros and expand by cofactors, or triangularize with Gauss.

See all the exercises in this section