Calculating Determinants
A fantasy-sports app records how four players scored in four games and stores the numbers in a $4 \times 4$ matrix. To test whether the data are redundant, you need its determinant. The diagonal shortcut you know for $3 \times 3$ matrices stops working at $4 \times 4$. In this lesson you will build tools that work for any size: triangular matrices, cofactor expansion and row reduction. You will also learn the properties that let you skip most of the arithmetic.
Quick review: $2 \times 2$ and $3 \times 3$
For a $2 \times 2$ matrix, multiply along the main diagonal and subtract the product along the other diagonal:
$$\begin{vmatrix} a & b \\ c & d \end{vmatrix} = ad - bc$$For a $3 \times 3$ matrix, use the diagonal method. Copy the first two columns to the right of the matrix (the shaded copies in the figure). Add the products of the three down-right diagonals, then subtract the products of the three up-right diagonals. Some problems call this shortcut Sarrus' rule.
A $2 \times 2$ determinant also measures area. The triangle with vertices $(0, 0)$, $(x_1, y_1)$ and $(x_2, y_2)$ has area
$$\text{Area} = \tfrac{1}{2}\left|\begin{vmatrix} x_1 & y_1 \\ x_2 & y_2 \end{vmatrix}\right|$$The absolute value keeps the area positive.
A streamer is analyzing the growth of their channel. To compare the impact of two of their videos, they organize the "Likes" and "Shares" data into a matrix $A$ of order…
A streamer is analyzing the growth of their channel. To compare the impact of two of their videos, they organize the "Likes" and "Shares" data into a matrix $A$ of order $2 \times 2$. Calculate the determinant of matrix $A$ to obtain an impact index:
$$A = \begin{pmatrix} 120 & 45 \\ 30 & 10 \end{pmatrix}$$Calculate the value of the determinant of matrix $B$ using Sarrus's rule. This matrix represents the inventory of components (chips, screens, and batteries) of three…
Calculate the value of the determinant of matrix $B$ using Sarrus's rule. This matrix represents the inventory of components (chips, screens, and batteries) of three warehouses of an electronics store:
$$B = \begin{pmatrix} 1 & 0 & 3 \\ 2 & 1 & 1 \\ 0 & 2 & 2 \end{pmatrix}$$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 $(1, 5)$.
$$Área = \frac{1}{2} \cdot \left| \begin{vmatrix} 4 & 2 \\ 1 & 5 \end{vmatrix} \right|$$Triangular matrices
A matrix is upper triangular if every entry below the main diagonal is $0$. It is lower triangular if every entry above the diagonal is $0$. A diagonal matrix counts as both.
A logistics company uses an upper triangular matrix to organize its delivery routes efficiently. Calculate the determinant of matrix $L$ knowing that for this type of…
A logistics company uses an upper triangular matrix to organize its delivery routes efficiently. Calculate the determinant of 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 the matrix equation $|X| = 0$, where $X = A - \lambda I$, with $A$ being the given matrix and $I$ the identity matrix of order 3. Find the values of $\lambda$ that…
Solve the matrix equation $|X| = 0$, where $X = A - \lambda I$, with $A$ being the given matrix and $I$ the identity matrix of order 3. Find the values of $\lambda$ that make the determinant zero (eigenvalues):
$$A = \begin{pmatrix} 1 & 0 & -1 \\ 0 & 2 & 3 \\ 0 & 0 & -1 \end{pmatrix}$$Minors and cofactors
The sign $(-1)^{i+j}$ is $+1$ when $i + j$ is even and $-1$ when it is odd. Picture a checkerboard that starts with $+$ in the top-left corner.
Given 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 matrix $M$, calculate its minor $\alpha_{21}$ and its cofactor $A_{21}$:
$$M = \begin{pmatrix} 2 & 1 & 3 \\ 4 & 0 & 5 \\ -1 & 2 & 1 \end{pmatrix}$$Cofactor expansion
Cofactor expansion reduces a $4 \times 4$ determinant to $3 \times 3$ determinants, which you already know how to compute.
Calculate the value of the following determinant of order 4 by expanding along the elements of the row or column you consider most appropriate (Laplace expansion):…
Calculate the value of the following determinant of order 4 by expanding along the elements of the row or column you consider most appropriate (Laplace expansion):
$$\Delta = \begin{vmatrix} 2 & 0 & 3 & 4 \\ 0 & 1 & -1 & 0 \\ 1 & 0 & 5 & 0 \\ 3 & 2 & 0 & 1 \end{vmatrix}$$Row operations
Another way to compute a determinant is to make zeros with row operations until the matrix is triangular, as in Gaussian elimination. Unlike solving a system, here you must track what each operation does to the determinant.
A useful trick is to add all the columns (or rows) together when they have a pattern, to create a common factor.
Calculate the value of the determinant of matrix $M$ of order 4 by previously transforming it into an upper triangular matrix using Gaussian elimination (making zeros…
Calculate the value of the determinant of matrix $M$ of order 4 by previously transforming it into an upper triangular matrix using Gaussian elimination (making zeros below the main diagonal):
$$M = \begin{pmatrix} 1 & 2 & 0 & 3 \\ -1 & 0 & 2 & 1 \\ 0 & 1 & -1 & 2 \\ 2 & 3 & 1 & 0 \end{pmatrix}$$Given 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…
Given 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 using Sarrus's rule, the value of the determinant of matrix $C$:
$$\begin{vmatrix} 3d & 3e & 3f \\ a+g & b+h & c+i \\ g & h & i \end{vmatrix}$$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 coordinate 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 fully simplified polynomial in $a$.
Properties of rows and columns
These properties let you see an answer without any computation.
Without performing complex calculations, justify why the following determinants are equal to $0$ using the properties of determinants: a)…
Without performing complex calculations, justify why the following determinants are equal to $0$ 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.
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:
$$| 3C_1 - C_2, \quad 2C_3, \quad C_2 + C_1 |$$Determinant of a product, power, transpose and inverse
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) $|3 \cdot A^t|$
c) $|A^{-1} \cdot (2A)|$
d) The determinant of the matrix resulting from swapping the first two rows of $A$ and multiplying the third row by $-5$.
Let matrix $A = (a_{ij})$ of order $3 \times 3$ be defined by $a_{ij} = i + j$. Also consider a matrix $B$ such that its determinant is $|B| = 2$. Using exclusively the…
Let matrix $A = (a_{ij})$ of order $3 \times 3$ be defined by $a_{ij} = i + j$. Also consider 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 matrix $M$, where:
$$M = (2A^t \cdot B^{-1})^2$$Determine whether the result obtained allows us to state that matrix $M$ has an inverse.
Determinant equations with a parameter
Many problems ask for the values of $x$ (or $k$, or $\lambda$) that make a determinant equal to $0$ or equal to another determinant. The plan is always the same: compute the determinant (using whichever method is quickest), get a polynomial in the unknown, then solve it.
The zeros of the determinant are the $x$-intercepts of its graph. Here is the polynomial $k^3 - 3k$ from the example (with $x$ standing for $k$):
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 $0$:
$$\begin{vmatrix} x & 8 \\ 2 & x \end{vmatrix} = 0$$Find all the real values of $x$ for which the following equality between determinants is satisfied: $$\begin{vmatrix} x & 1 & 1 \\ 1 & x & 1 \\ 1 & 1 & x \end{vmatrix} =…
Find all the real values of $x$ for which the following equality between determinants is satisfied:
$$\begin{vmatrix} x & 1 & 1 \\ 1 & x & 1 \\ 1 & 1 & x \end{vmatrix} = \begin{vmatrix} x & -1 \\ 2 & 1 \end{vmatrix}$$Determine for which values of the real parameter $k$ matrix $A$ does not have an inverse. To do this, equate its determinant to zero and solve the resulting equation:…
Determine for which values of the real parameter $k$ matrix $A$ does not have an inverse. To do this, equate its determinant to zero and solve the resulting equation:
$$A = \begin{pmatrix} k & 1 & 2 \\ 2 & k+1 & 0 \\ 1 & k & 1 \end{pmatrix}$$Patterns: Vandermonde and constant row sums
Some determinants factor in a beautiful way. Recognizing the pattern saves a lot of work.
Check with numbers: for $a, b, c = 2, 3, 5$ the formula gives $(3-2)(5-2)(5-3) = 6$. Direct expansion of $\begin{vmatrix} 1 & 2 & 4 \\ 1 & 3 & 9 \\ 1 & 5 & 25 \end{vmatrix}$ gives $30 - 32 + 8 = 6$.
Another pattern: if every row adds up to the same number, add all the columns into one. The first column becomes a constant, which you factor out. For example, if each row of a $3 \times 3$ matrix adds up to $s$, then $s$ is a factor of the determinant. Then use row operations on the remaining $1$'s, as in the $b+2$ example in the row-operations section.
In the development of a strategy video game, the position of three units on the map is given by the rows of a matrix. The determinant of this matrix indicates the…
In the development of a strategy video game, the position of three units on the map is given by the rows of a matrix. The determinant of this matrix indicates the battalion's area of influence. Calculate the value of $a$ so that the area of influence is zero (aligned or linearly dependent units) in the following system:
$$\begin{vmatrix} 1 & a & a^2 \\ 1 & 2 & 4 \\ 1 & -3 & 9 \end{vmatrix} = 0$$Note: This is a Vandermonde determinant. Try to simplify it using column properties before calculating it.
In a telecommunications network, the flow capacity is modeled by the determinant of the adjacency matrix $W$. $W$ is defined as: $$W = \begin{vmatrix} 1 & a & a^2 \\ 1 &…
In a telecommunications network, the flow capacity is modeled by 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) Prove, without expanding the determinant using Sarrus's rule, that $W = (b-a)(c-a)(c-b)$.
- b) If the nodes $a, b, c$ represent transmission frequencies, what condition must they meet for the determinant to be non-zero? Justify the importance of this condition for the independence of the signals.
Solve the following cubic equation using the properties of determinants to factor the resulting polynomial: $$\begin{vmatrix} x & 2 & 3 \\ 2 & x & 3 \\ 2 & 3 & x…
Solve the following cubic equation using the properties of determinants to factor the resulting polynomial:
$$\begin{vmatrix} x & 2 & 3 \\ 2 & x & 3 \\ 2 & 3 & x \end{vmatrix} = 0$$Indicate step by step the elementary operations performed on the rows or columns.
Invertibility, rank, the adjugate and matrix equations
The determinant answers several questions about a square matrix $A$ at once. These statements are all equivalent:
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$|A| \ne 0$;
-
$A$ has an inverse;
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the rows (and columns) are linearly independent;
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$A$ has full rank $n$.
The adjugate
The adjugate $\text{adj}(A)$ is the transpose of the matrix of cofactors of $A$. It satisfies
$$A\cdot\text{adj}(A) = |A|\,I$$Taking determinants of both sides gives $|A|\cdot|\text{adj}(A)| = |A|^n$, so for an $n \times n$ matrix
$$|\text{adj}(A)| = |A|^{\,n-1}$$This formula also holds when $|A| = 0$. For a $3 \times 3$ matrix, $|\text{adj}(A)| = |A|^2$. For example, if $|A| = 4$ then $|\text{adj}(A)| = 16$, and you never have to compute a single cofactor.
Matrix equations
To get $|X|$ from an equation like $AXA^T = I$, take determinants of both sides:
$$|A|\cdot|X|\cdot|A^T| = |I| = 1 \quad\Rightarrow\quad |A|^2\,|X| = 1 \quad\Rightarrow\quad |X| = \frac{1}{|A|^2}.$$For example, if $|A| = -3$ then $|X| = \tfrac{1}{9}$. To find $X$ itself, multiply by inverses on both sides: $X = A^{-1}(A^T)^{-1}$. You can find the inverse with Gaussian elimination, or with $A^{-1} = \dfrac{\text{adj}(A)}{|A|}$ (next lesson).
Back to the opening: for a $4 \times 4$ matrix of player data, you can expand along a row or column with many zeros, or reduce to a triangular matrix while keeping track of swaps and scalings. If the determinant is $0$, the data are redundant.
In the design of an aeronautical component, the structural stability is given by the determinant of a stress matrix $T$. It is known that the determinant of the…
In the design of an aeronautical component, the structural stability is given by the determinant of a stress matrix $T$. It is known that the determinant of the following matrix depends on a load parameter $k$:
$$|T| = \begin{vmatrix} 1 & 1 & 1 \\ k & 2 & 1 \\ k^2 & 4 & 1 \end{vmatrix}$$- a) Calculate the value of the determinant in terms of $k$ using the properties of determinants to simplify the calculation (avoid direct expansion using Sarrus's rule).
- b) Determine for which values of $k$ the component is unstable ($|T| = 0$).
- c) If $k=3$, what relationship exists between the rows of the matrix?
Given the matrix dependent on a real parameter $\lambda$: $$A(\lambda) = \begin{pmatrix} \lambda & 1 & 2 \\ 1 & \lambda & 0 \\ \lambda & 1 & \lambda \end{pmatrix}$$ a)…
Given the matrix dependent on a real parameter $\lambda$:
$$A(\lambda) = \begin{pmatrix} \lambda & 1 & 2 \\ 1 & \lambda & 0 \\ \lambda & 1 & \lambda \end{pmatrix}$$- a) Find the values of $\lambda$ for which matrix $A(\lambda)$ does not have an inverse.
- b) For $\lambda = 0$, solve the matrix equation $A \cdot X \cdot A^t = I$, where $I$ is the identity matrix of order 3. Calculate $|X|$ indirectly before finding matrix $X$.
Analyze the rank of the following matrix $A$ in terms of the values of the real parameters $m$ and $n$: $$A = \begin{pmatrix} 1 & 0 & 2 \\ m & 0 & n \\ 3 & -1 & 4…
Analyze the rank of the following matrix $A$ in terms of 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 previously calculating the elements of the adjugate matrix. (Hint: use the relationship between a matrix, its adjugate, and its determinant).