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Determinants · Determinants

Rank of a Matrix

Before you start, make sure you know: Factoring polynomials (High School Year 2 (semesters 3 and 4))
Before you start, make sure you know: Linear combinations of vectors. Bases (High School Year 2 (semesters 3 and 4))

Ximena keeps the books for a stationery store with three branches. In a matrix she records, one row per branch, the weekly sales of each branch for four products. One day she notices that the sales of the third branch are exactly the sum of the other two. How many rows really provide new information? That is the question the rank of a matrix answers, and determinants are the tool for computing it.

Review: determinants of order 2 and 3

The determinant exists only for square matrices. It is written $\det A$ or $|A|$.

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Calculate the determinant of the following matrix $A$ of order 2. This value represents the scale factor of a geometric transformation in the plane: $$A =…

Calculate the determinant of the following matrix $A$ of order 2. This value represents the scale factor of a geometric transformation in the plane:

$$A = \begin{pmatrix} 5 & -2 \\ 3 & 4 \end{pmatrix}$$ Matrix A A = 5 3 −2 4 det(A) = ? Geometric Transformation x y Area = ? v₁ v₂
Solve
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Given the matrix $M$, calculate the value of the real parameter $x$ so that its determinant is equal to $10$: $$M = \begin{pmatrix} x & 4 \\ 1 & 2 \end{pmatrix}$$

Given the matrix $M$, calculate the value of the real parameter $x$ so that its determinant is equal to $10$:

$$M = \begin{pmatrix} x & 4 \\ 1 & 2 \end{pmatrix}$$
Solve
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Find the determinant of the matrix $B$ using Sarrus' rule: $$B = \begin{pmatrix} 1 & 0 & 3 \\ 2 & 1 & -1 \\ 0 & 4 & 2 \end{pmatrix}$$

Find the determinant of the matrix $B$ using Sarrus' rule:

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

Cofactors and triangular matrices

The factor $(-1)^{i+j}$ gives a checkerboard sign pattern: $\begin{pmatrix}+&-&+\\-&+&-\\+&-&+\end{pmatrix}$.

For example, $\begin{vmatrix}2&5&1\\0&-3&4\\0&0&6\end{vmatrix}=2\cdot(-3)\cdot6=-36$.

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In the matrix $C$, identify the element $a_{23}$ and calculate its cofactor $A_{23}$: $$C = \begin{pmatrix} 2 & 1 & 5 \\ 0 & -1 & 3 \\ 4 & 2 & 1 \end{pmatrix}$$

In the matrix $C$, identify the element $a_{23}$ and calculate its cofactor $A_{23}$:

$$C = \begin{pmatrix} 2 & 1 & 5 \\ 0 & -1 & 3 \\ 4 & 2 & 1 \end{pmatrix}$$
Solve
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Quickly calculate the determinant of the following upper triangular matrix, giving reasons for your answer: $$T = \begin{pmatrix} 3 & 7 & 9 \\ 0 & -2 & 1 \\ 0 & 0 & 5…

Quickly calculate the determinant of the following upper triangular matrix, giving reasons for your answer:

$$T = \begin{pmatrix} 3 & 7 & 9 \\ 0 & -2 & 1 \\ 0 & 0 & 5 \end{pmatrix}$$
Solve

Properties that save work

These properties hold equally for rows and for columns.

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Given that the determinant of a square matrix $A$ of order 2 is $|A| = 5$, what is the value of the determinant of the matrix $2A$? Apply the properties of determinants…

Given that the determinant of a square matrix $A$ of order 2 is $|A| = 5$, what is the value of the determinant of the matrix $2A$? Apply the properties of determinants to justify your answer.

Solve
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Knowing that the value of the determinant of the matrix $3 \times 3$ formed by the column vectors $\vec{c_1}$, $\vec{c_2}$ and $\vec{c_3}$ is equal to $5$, that is,…

Knowing that the value of the determinant of the matrix $3 \times 3$ formed by the column vectors $\vec{c_1}$, $\vec{c_2}$ and $\vec{c_3}$ is equal to $5$, that is, $|\vec{c_1}, \vec{c_2}, \vec{c_3}| = 5$, calculate without expanding the value of:

$|\vec{c_1} + \vec{c_3}, \quad 2\vec{c_2}, \quad \vec{c_1} - \vec{c_3}|$

Justify each step mentioning the property of determinants that you have applied.

Solve
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Consider the square matrix $A$ of order 3 whose columns are $C_1, C_2, C_3$. It is known that $|A| = 5$. Calculate the value of the determinant of the matrix $M$ whose…

Consider the square matrix $A$ of order 3 whose columns are $C_1, C_2, C_3$. It is known that $|A| = 5$. Calculate the value of the determinant of the matrix $M$ whose columns are:

$$M = (C_1 + C_2, \quad C_2 + C_3, \quad C_3 + C_1)$$

If we replace the third column of $M$ with the zero vector, what would be the rank of the new matrix? Justify your answer without performing detailed numerical calculations.

Solve

Creating zeros to compute large determinants

For order 4 or more, the practical approach is to use property 5 to create zeros in a column (it is the same thing you do in Gaussian elimination) and then expand by cofactors.

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Calculate the value of the following determinant of order $4$ using Gaussian elimination to create zeros in a row or column before expanding by cofactors:…

Calculate the value of the following determinant of order $4$ using Gaussian elimination to create zeros in a row or column before expanding by cofactors:

$|D| = \begin{vmatrix} 1 & 2 & 0 & 3 \\ -1 & 3 & 1 & 4 \\ 2 & 0 & -2 & 1 \\ 0 & 1 & 3 & 2 \end{vmatrix}$

1 2 0 3 -1 3 1 4 2 0 -2 1 0 1 3 2 F₂ → F₂ + F₁ F₃ → F₃ - 2F₁ 1 2 0 3 0 5 1 7 0 -4 -2 -5 0 1 3 2 4x4 Determinant Calculation (Gauss + Cofactors) 1. Choose pivot 2. Create zeros 3. By cofactors 1 2 0 3 -1 3 1 4 2 0 -2 1 0 1 3 2 F₂+F₁ F₃-2F₁ Gauss 1 2 0 3 0 ? ? ? 0 ? ? ? 0 1 3 2 = 1· ? ? ? ? ? ? 1 3 2 = ?
Solve
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Calculate, without expanding directly by Sarrus' rule, the value of the following determinant of order 4, applying elementary transformations to create zeros and…

Calculate, without expanding directly by Sarrus' rule, the value of the following determinant of order 4, applying elementary transformations to create zeros and simplify the calculation:

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

What condition must $a, b$ and $c$ satisfy for the rank of the associated matrix to be exactly 2?

Solve

Determinant of a product, of the inverse, and of the adjugate

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Given a square matrix $A$ of order $3$ whose determinant is $|A| = 4$, use the properties of determinants to calculate with reasoning the value of the following…

Given a square matrix $A$ of order $3$ whose determinant is $|A| = 4$, use the properties of determinants to calculate with reasoning the value of the following determinants:

  1. a) $|A^t \times A^{-1}|$
  1. b) $|2 \times A \times A^t|$
  1. c) The determinant of the matrix resulting from swapping the first two rows of $A$ and multiplying the third column by $-3$.
Solve
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Given the $3 \times 3$ matrices $A$ and $B$, you know that $A$ has rank $3$ and that the determinant of $B$ is $|B| = 0$. a) What is the value of the determinant of the…

Given the $3 \times 3$ matrices $A$ and $B$, you know that $A$ has rank $3$ and that the determinant of $B$ is $|B| = 0$.

  1. a) What is the value of the determinant of the product $|A \times B|$? Justify your answer.
  1. b) Is it possible for the rank of the product matrix $A \times B$ to be $3$? Explain why, based on the relationship between the determinant and the rank.
Solve
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Let $A$ be a square matrix of order $n=4$ such that its determinant is $|A| = \frac{1}{2}$. Using only the properties of determinants, calculate the value of the…

Let $A$ be a square matrix of order $n=4$ such that its determinant is $|A| = \frac{1}{2}$. Using only the properties of determinants, calculate the value of the determinant of the matrix $B$, defined as:

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

where $A^t$ is the transpose of $A$ and $Adj(A)$ is its adjugate matrix. Justify each step of your calculation, indicating the property applied.

Solve

What the rank measures

A row that is a multiple of another, or a sum of others, does not add a new direction: it is "redundant" for the rank. This is the same thing you get when you row reduce: the rank is the number of nonzero rows in the echelon form.

Three rows of a matrix; the second is twice the first, so the rank is 2. R1 = (1, 2, 3) R2 = (2, 4, 6) = 2·R1 R3 = (0, 1, 1) repeated new

The matrix has 3 rows but rank $2$.

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Determine the rank of the following matrix $A$ by observing the relationship between its rows: $$A = \begin{pmatrix} 1 & 2 & 3 \\ 2 & 4 & 6 \\ 0 & 1 & 1 \end{pmatrix}$$

Determine the rank of the following matrix $A$ by observing the relationship between its rows:

$$A = \begin{pmatrix} 1 & 2 & 3 \\ 2 & 4 & 6 \\ 0 & 1 & 1 \end{pmatrix}$$
Solve

Rank using minors

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Calculate the rank of the matrix $B$ by finding the highest-order minor that is nonzero: $$B = \begin{pmatrix} 1 & 0 & 5 \\ -2 & 1 & 3 \end{pmatrix}$$ a) State the…

Calculate the rank of the matrix $B$ by finding the highest-order minor that is nonzero:

$$B = \begin{pmatrix} 1 & 0 & 5 \\ -2 & 1 & 3 \end{pmatrix}$$
  1. a) State the highest possible order of a minor of $B$ and how many minors of that order can be formed.
  1. b) Calculate one of those minors and check whether it is nonzero.
  1. c) Write the rank of $B$ and justify your answer.
Solve

$3\times4$ matrices: bordered minors

To avoid trying minors blindly: start from a small nonzero minor and border it, that is, add one row and one column to it. If all the borderings equal $0$, the rank does not go up.

By rows you arrive at the same result: $R_3-R_1-R_2=(0,0,0,k-5)$.

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Determine the rank of the matrix $B$ according to the values of the real parameter $a$:…

Determine the rank of the matrix $B$ according to the values of the real parameter $a$:

$B = \begin{pmatrix} 1 & a & 1 & 2 \\ 2 & -1 & a & 1 \\ 1 & 10 & -3 & 5 \end{pmatrix}$

Remember that for a matrix of dimensions $3 \times 4$, the maximum possible rank is $3$.

Solve
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A robotics engineer is programming the movement of a mechanical arm in three-dimensional space. The sensors detect three force vectors that determine the stability of…

A robotics engineer is programming the movement of a mechanical arm in three-dimensional space. The sensors detect three force vectors that determine the stability of the system. The matrix representing these forces is:

$$M = \begin{pmatrix} k & 1 & 1 & 0 \\ 1 & k & 1 & 2 \\ 1 & 1 & k & k \end{pmatrix}$$

You are asked to:

  1. Determine the rank of the matrix $M$ in terms of the real parameter $k$.

  2. Is there any value of $k$ for which the forces are linearly dependent but the rank of the matrix is maximum? Explain your answer based on the theory of minors.

Solve

Rank according to a parameter

When the matrix depends on a letter $k$, the procedure is always the same: compute the largest possible determinant, factor it, find its roots, and at each root analyze the rank using smaller minors.

Number line with the value k equal to minus 1, where the determinant vanishes; to the left and to the right the rank is 3 and at that value it is 2. k = −1 k < −1: rg = 3 k = −1: rg = 2 k > −1: rg = 3

The parameter can also be $\lambda$, as in $\det(A-\lambda I)$. And sometimes the determinant does not depend on the variable: in $\begin{vmatrix}\cos x&-\sin x\\\sin x&\cos x\end{vmatrix}=\cos^2x+\sin^2x=1$ for every $x$.

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Determine the rank of the following matrix $A$ depending on the possible values of the real parameter $k$:…

Determine the rank of the following matrix $A$ depending on the possible values of the real parameter $k$:

$A = \begin{pmatrix} k & 1 & 1 \\ 1 & k & 1 \\ 1 & 1 & k \end{pmatrix}$

Determine for which values of $k$ the rank is $1$, $2$ or $3$.

k k₁ = ? k₂ = ? A = k 1 1 1 k 1 1 1 k rg(A) = 1 ? rg(A) = 2 ? rg(A) = 3 ? |A| = 0 k = ? rg(A) = ? rg(A) = ? rg(A) = ? rg(A) = ? rg(A) = ?
Solve
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Study the linear dependence of the rows of the matrix $A$ according to the values of the real parameters $a$ and $b$: $$A = \begin{pmatrix} 1 & a & b \\ a & 1 & b \\ a &…

Study the linear dependence of the rows of the matrix $A$ according to the values of the real parameters $a$ and $b$:

$$A = \begin{pmatrix} 1 & a & b \\ a & 1 & b \\ a & b & 1 \end{pmatrix}$$

Determine for which values of $(a, b)$ the rank of $A$ is equal to 1. Describe geometrically what relationship the row vectors have when this occurs.

Solve
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Given the square matrix of order 3: $$A = \begin{pmatrix} \cos x & -\sin x & 0 \\ \sin x & \cos x & 0 \\ 0 & 0 & 1 \end{pmatrix}$$ 1. Show that $|A|$ is independent of…

Given the square matrix of order 3:

$$A = \begin{pmatrix} \cos x & -\sin x & 0 \\ \sin x & \cos x & 0 \\ 0 & 0 & 1 \end{pmatrix}$$
  1. Show that $|A|$ is independent of the value of the variable $x$.

  2. If we define a new matrix $B = A - \lambda I$ (where $I$ is the identity), find the values of $\lambda$ for which the rank of $B$ is less than 3 when $x = \frac{\pi}{4}$.

Solve

Square matrices: rank, determinant, and inverse

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In the design of a graphics engine for a video game, a transformation matrix $M$ is required to be invertible in order to undo movements. Given the matrix:…

In the design of a graphics engine for a video game, a transformation matrix $M$ is required to be invertible in order to undo movements. Given the matrix:

$M = \begin{pmatrix} 1 & 0 & -1 \\ 0 & m & 3 \\ 4 & 1 & -m \end{pmatrix}$

  1. a) Find the values of the real parameter $m$ for which the matrix $M$ does not possess an inverse.
  1. b) For the value $m = 2$, calculate the value of the determinant of the inverse matrix, $|M^{-1}|$.
Solve
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Solve the following quadratic equation expressed in determinant form: $\begin{vmatrix} x & 1 & 3 \\ 1 & x & 2 \\ 3 & 2 & 1 \end{vmatrix} = 0$ Find the exact solutions…

Solve the following quadratic equation expressed in determinant form:

$\begin{vmatrix} x & 1 & 3 \\ 1 & x & 2 \\ 3 & 2 & 1 \end{vmatrix} = 0$

Find the exact solutions (they may contain roots).

Solve
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Given the matrix: $$A = \begin{pmatrix} \alpha & 1 & 1 \\ 1 & \alpha & 1 \\ 1 & 1 & \alpha \end{pmatrix}$$ 1. Find the values of $\alpha$ for which the matrix does not…

Given the matrix:

$$A = \begin{pmatrix} \alpha & 1 & 1 \\ 1 & \alpha & 1 \\ 1 & 1 & \alpha \end{pmatrix}$$
  1. Find the values of $\alpha$ for which the matrix does not have an inverse.

  2. For the values found in the previous section, calculate the rank of the matrix $A^2$.

  3. Is it possible to find a value of $\alpha$ such that $rg(A) \neq rg(A^2)$? Justify your answer based on the properties of rank and determinants.

Solve

The Vandermonde determinant

The determinant is nonzero exactly when $x$, $y$, $z$ are all different from each other. With a parameter $m$ and fixed values $a$, $b$, the rank of $\begin{pmatrix}1&m&m^2\\1&a&a^2\\1&b&b^2\end{pmatrix}$ is $3$ except when $m=a$ or $m=b$.

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In a telecommunications system, the signal is lost if the determinant of the transmission matrix vanishes. The transmission matrix is given by: $$T = \begin{pmatrix} 1 &…

In a telecommunications system, the signal is lost if the determinant of the transmission matrix vanishes. The transmission matrix is given by:

$$T = \begin{pmatrix} 1 & m & m^2 \\ 1 & a & a^2 \\ 1 & b & b^2 \end{pmatrix}$$
  1. a) Knowing that $|T|$ is a Vandermonde determinant, express it as a product of linear factors.
  1. b) If we set $a=2$ and $b=5$, discuss the rank of $T$ according to the values the parameter $m$ can take. For which values of $m$ is the rank minimal, and what is that minimum rank?
Solve

Back to the beginning: if the third row of Ximena's sales is the sum of the other two, the rank is 2: the third branch adds no new information, and any $3\times3$ determinant of those rows equals $0$.

Summary

See all the exercises in this section