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
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:
$$A = \begin{pmatrix} 120 & 45 \\ 30 & 10 \end{pmatrix}$$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 $(1, 5)$.
Determinant of a triangular matrix
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:
$$L = \begin{pmatrix} 5 & -2 & 1 \\ 0 & 3 & 4 \\ 0 & 0 & 2 \end{pmatrix}$$Minors and cofactors
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 $A_{21}$:
$$M = \begin{pmatrix} 2 & 1 & 3 \\ 4 & 0 & 5 \\ -1 & 2 & 1 \end{pmatrix}$$Expansion along a row or column: Laplace
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:
$$B = \begin{pmatrix} 1 & 0 & 3 \\ 2 & 1 & 1 \\ 0 & 2 & 2 \end{pmatrix}$$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.
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:
- a) $D_1 = \begin{vmatrix} 1 & 2 & 3 \\ 0 & 0 & 0 \\ 5 & 1 & 4 \end{vmatrix}$
- b) $D_2 = \begin{vmatrix} 2 & -1 & 4 \\ 2 & -1 & 4 \\ 10 & 20 & 30 \end{vmatrix}$
- c) $D_3 = \begin{vmatrix} 1 & 3 & 2 \\ 2 & 6 & 4 \\ 0 & 1 & 5 \end{vmatrix}$
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.
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 $-5$.
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.
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 $C$:
$$\begin{vmatrix} 3d & 3e & 3f \\ a+g & b+h & c+i \\ g & h & i \end{vmatrix}$$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:
$$| 3C_1 - C_2, \quad 2C_3, \quad C_2 + C_1 |$$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.
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 $\lambda$:
$$A(\lambda) = \begin{pmatrix} \lambda & 1 & 2 \\ 1 & \lambda & 0 \\ \lambda & 1 & \lambda \end{pmatrix}$$- a) Find the values of $\lambda$ for which the matrix $A(\lambda)$ does not have an inverse.
- 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 $X$.
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.
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 $0$:
$$\begin{vmatrix} x & 8 \\ 2 & x \end{vmatrix} = 0$$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 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$$State step by step the elementary operations performed on the rows or columns.
Vandermonde-type determinants
Some determinants with successive powers ($1, a, a^2$) can be factored using row operations, without Sarrus. The technique: subtract the first row from the others to create zeros and factor out common factors.
In the development of a strategy video game, the positions of three units on the map are given by the rows of a matrix. The determinant of this matrix indicates the…
In the development of a strategy video game, the positions of three units on the map are given by the rows of a matrix. The determinant of this matrix indicates the battalion's area of influence. Calculate the value of $a$ so that the area of influence is zero (units collinear or linearly dependent) in the following system:
$$\begin{vmatrix} 1 & a & a^2 \\ 1 & 2 & 4 \\ 1 & -3 & 9 \end{vmatrix} = 0$$Note: This is a Vandermonde determinant. Try to simplify it using column properties before calculating it.
In the design of an aeronautical component, structural stability is given by the determinant of a stress matrix $T$. It is known that the determinant of the following…
In the design of an aeronautical component, structural stability is given by the determinant of a stress matrix $T$. It is known that the determinant of the following matrix depends on a load parameter $k$:
$$|T| = \begin{vmatrix} 1 & 1 & 1 \\ k & 2 & 1 \\ k^2 & 4 & 1 \end{vmatrix}$$- a) Calculate the value of the determinant in terms of $k$ using the properties of determinants to simplify the calculation (avoid direct expansion by Sarrus' rule).
- b) Determine for which values of $k$ the component is unstable ($|T| = 0$).
- c) If $k=3$, what relationship exists between the rows of the matrix?
In a telecommunications network, flow capacity is modelled using the determinant of the adjacency matrix $W$. $W$ is defined as: $$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:
$$W = \begin{vmatrix} 1 & a & a^2 \\ 1 & b & b^2 \\ 1 & c & c^2 \end{vmatrix}$$- a) Show that, without expanding the determinant by Sarrus' rule, $W = (b-a)(c-a)(c-b)$.
- 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.
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 $A$.
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):
$$A = \begin{pmatrix} 1 & 0 & -1 \\ 0 & 2 & 3 \\ 0 & 0 & -1 \end{pmatrix}$$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}$$Extension: rank by minors
The rank of a matrix is the largest order of a minor (determinant of a square submatrix) that is nonzero. So a $3\times3$ matrix has rank $3$ if $|A|\neq 0$. If $|A|=0$, you look for a nonzero minor of order 2: if one exists, the rank is $2$; if they are all $0$ but some entry is nonzero, the rank is $1$.
Determine the rank of the following matrix $A$ according to the values of the real parameters $m$ and $n$: $$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).
Extension: order 4 determinants with Laplace
Laplace works for any order: a $4\times4$ determinant reduces to $3\times3$ determinants (and those, with Sarrus).
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}$$A GPS positioning system uses four satellites to triangulate a position in space. The volume of the tetrahedron formed by the four satellites is related to the absolute…
A GPS positioning system uses four satellites to triangulate a position in space. The volume of the tetrahedron formed by the four satellites is related to the absolute value of the determinant of their coordinates matrix. Calculate with reasoning the value of the following determinant of order 4, which represents a specific configuration of these satellites:
$$\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}$$Express the result as a polynomial in $a$ simplified as far as possible.
Extension: triangularize with Gauss before calculating
Calculate the value of the determinant of the matrix $M$ of order 4 by first transforming it into an upper triangular matrix using Gaussian elimination (creating zeros…
Calculate the value of the determinant of the matrix $M$ of order 4 by first transforming it into an upper triangular matrix using Gaussian elimination (creating zeros below the main diagonal):
$$M = \begin{pmatrix} 1 & 2 & 0 & 3 \\ -1 & 0 & 2 & 1 \\ 0 & 1 & -1 & 2 \\ 2 & 3 & 1 & 0 \end{pmatrix}$$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.