Back to homeMathematics · Secondary Year 5 · Change subject
Start for free
Secondary 5 · Mathematics
Determinants · Determinants

Calculating Determinants

Yaritza works at a hardware store in La Vega and keeps her inventory in a table with 3 rows and 3 columns, with several zeros. Her teacher asks her for the determinant, and she thinks all she can do is apply Sarrus with its six multiplications. Is there a shorter way? Yes: take advantage of the zeros, break the determinant down into smaller ones (Laplace), or use properties to simplify before calculating.

Review: order 2 determinant

Practice★☆☆Not started
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
Practice★☆☆Not started
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

Determinant of a triangular matrix

Upper triangular matrix of order 3 with the zeros shaded and its determinant obtained as the product of the diagonal. 5 −2 1 0 3 4 0 0 2 |A| = 5 · 3 · 2 = 30
Practice★☆☆Not started
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

Minors and cofactors

Matrix of order 3 with row 2 and column 1 highlighted and the entry a₂₁ marked: what is not covered forms the minor of a₂₁. a₁₁ a₁₂ a₁₃ a₂₁ a₂₂ a₂₃ a₃₁ a₃₂ a₃₃ cover row 2 and column 1 (the ones through a₂₁)
Practice★☆☆Not started
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

Expansion along a row or column: Laplace

Matrix of order 3 with a row containing two zeros highlighted: in the Laplace expansion only one product survives. 203 010 105 row with two zeros: only 1 product remains
Practice★☆☆Not started
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

Properties that save work

Before calculating, check whether the properties give you the result.

Why the last one? With cofactors, $A\cdot\operatorname{Adj}(A)^t = |A|\,I$ holds. Taking determinants: $|A|\cdot|\operatorname{Adj}(A)| = |A|^n$, and from there $|A|^{n-1}$ follows. This way you can calculate $|\operatorname{Adj}(A)|$ without building the adjugate.

Practice★☆☆Not started
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
Practice★☆☆Not started
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
Practice★★☆Not started
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

Combining properties

Many problems give you $|A|$ (or the columns of $A$) and ask for the determinant of another matrix. The idea is to turn the new matrix into the original one using operations whose effect you know.

Practice★★☆Not started
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
Practice★★★Not started
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
Practice★★★Not started
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
Practice★★★Not started
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

Determinants with parameters

When there is an unknown, you calculate the determinant (with Sarrus, properties or Laplace) and get a polynomial. Setting it equal to zero gives the values that make it vanish, and for those values the matrix has no inverse.

Practice★☆☆Not started
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
Practice★★☆Not started
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
Practice★★★Not started
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

Vandermonde-type determinants

Some determinants with successive powers ($1, a, a^2$) can be factored using row operations, without Sarrus. The technique: subtract the first row from the others to create zeros and factor out common factors.

Practice★★☆Not started
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
Practice★★★Not started
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
Practice★★★Not started
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

Eigenvalues: $|A-\lambda I|=0$

If you subtract an unknown $\lambda$ from the diagonal of $A$, you get $A-\lambda I$, and its determinant is a polynomial in $\lambda$. Solving $|A-\lambda I|=0$ means solving an equation with a parameter: its solutions are called the eigenvalues of $A$.

Practice★★☆Not started
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
Practice★★☆Not started
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

Extension: rank by minors

The rank of a matrix is the largest order of a minor (determinant of a square submatrix) that is nonzero. So a $3\times3$ matrix has rank $3$ if $|A|\neq 0$. If $|A|=0$, you look for a nonzero minor of order 2: if one exists, the rank is $2$; if they are all $0$ but some entry is nonzero, the rank is $1$.

Practice★★★Not started
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

Extension: order 4 determinants with Laplace

Laplace works for any order: a $4\times4$ determinant reduces to $3\times3$ determinants (and those, with Sarrus).

Practice★★☆Not started
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
Practice★★★Not started
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

Extension: triangularize with Gauss before calculating

A matrix of order 3 is transformed by subtracting three times row 1 from row 2, which does not change the determinant. 120 314 012 F₂ − 3F₁ → 120 0−54 012 the determinant does not change
Practice★★☆Not started
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

Summary

Go back to the beginning: Yaritza doesn't need the six multiplications of Sarrus. She can expand by Laplace along the row or column with the most zeros, or use the properties to simplify before calculating.

See all the exercises in this section