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determinants and inverse matrices

Module - 1 Linear Algebra and Vector Algebra

Lesson - 4 Determinants and Inverse Matrices

**Overview**

Determinants and reverse networks are vital concepts in linear algebra, which may be a department of arithmetic that deals with vectors, lattices, and straight transformations. At the conclusion of this lesson, we are going to learn what are determinants and reverse of networks in detail, and we are going check few illustrations to understand the concept with more clarity.

**Definition and properties of determinants:**

A determinant is a scalar value that can be computed from a square matrix, which encodes information about the linear transformation represented by the matrix. The determinant of an n x n matrix A is denoted by det(A) or |A| and can be defined recursively as follows:

- For a 1 x 1 matrix, det(A) = a11.
- For an n x n matrix where n > 1,

```
det(A) = (-1)^(i+1) * a1i * det(A1i)
```

where i is any integer from 1 to n, a1i is the element in the first row and ith column of A, and A1i is the (n-1) x (n-1) matrix obtained by deleting the first row and ith column of A.

**Some key properties of determinants include:**

**Linearity**: If A and B are matrices of the same size, and k is a scalar, then

```
det(kA) = k^n * det(A) and det(A + B) = det(A) + det(B)
```

- Swapping two rows or columns of a matrix changes the sign of its determinant: If A' is obtained from A by swapping two rows or columns, then

```
det(A') = -det(A)
```

- If A is a singular matrix (i.e., it does not have an inverse), then det(A) = 0.

For example, consider the matrix A = [1 2; 3 4]. The determinant of A is given by:

```
det(A) = (-1)^(1+1) * 1 * det([4]) - (-1)^(1+2) * 2 * det([3]) = 4 - 6 = -2
```

Thus, the determinant of A is -2. We can also verify this using the formula

```
det(A) = ad - bc
```

where a, b, c, and d are the entries of A:

```
det(A) = 14 - 23 = -2
```

**Computing determinants:**

There are several methods for computing determinants, including cofactor expansion and row reduction. Here, we'll focus on cofactor expansion:

- Choose a row or column of A to expand along.
- For each element aij in that row or column, compute its corresponding cofactor Cij, which is given by

```
Cij = (-1)^(i+j) * det(Aij)
```

where Aij is the (n-1) x (n-1) matrix obtained by deleting row i and column j from A.

- Multiply each element in the chosen row or column by its corresponding cofactor, and then sum the products to get det(A).

For example, let's compute the determinant of the matrix

```
A = [1 2 3]
[4 5 6]
[7 8 9]
```

by expanding along the first row:

```
det(A) = 1C11 + 2C12 + 3C13,
where C11 = det([5 6]
[8 9]) = 59 - 68 = -3,
C12 = det([4 6]
[7 9]) = 49 - 67 = 6,
and C13 = det([4 5]
[7 8]) = 48 - 5*7 = -3
Therefore, det(A) = 1*(-3) + 26 + 3(-3) = 0
```

**Inverse matrices and their relationship to determinants:**

An inverse matrix is a matrix that can be multiplied by the original matrix to obtain the identity matrix, which represents the identity transformation. The inverse matrix can be computed using the determinant and the adjugate of the matrix. Singular matrices, which have zero determinants, have important implications in linear algebra and applications.

The inverse of an n x n matrix A is denoted by A^-1 and is defined as the unique matrix such that

```
A * A^(-1) = A^(-1) * A = I
```

where I is the identity matrix of size n x n.

- Not all matrices have an inverse, and those that do are called
**invertible or non-singular matrices**. - The determinant plays an important role in determining whether a
**matrix****is****invertible**. Specifically, a matrix A is invertible if and only if**det(A) ≠ 0**. In this case, the inverse of A can be computed as:

```
A^-1 = (1/det(A)) * adj(A)
```

where adj(A) is the adjugate (or classical adjoint) of A, which is the transpose of the matrix of cofactors of A.

For example, consider the matrix

```
A = [2 1]
[4 3]
```

The determinant of A is **det(A) = 2****3 - 1****4 = 2**, which is non-zero. Therefore, A is invertible, and its **inverse** is given by:

```
A^-1 = (1/det(A)) * adj(A) = (1/2) * [3 -1] = [3/2 -1/2]
[-4 2] [ -2 1]
```

We can verify that A^-1 * A = I and A * A^-1 = I, where

I = [1 0]

[0 1]:

```
A^-1 * A = [3/2 -1/2] * [2 1] = [1 0] = I
[ -2 1] [4 3] [0 1]
```

and

```
A * A^-1 = [2 1] * [3/2 -1/2] = [1 0] = I
[4 3] [ -2 1] [0 1]
```

This confirms that A^-1 is indeed the inverse of A.

**Note that not all matrices are invertible.** For example, the matrix

```
B = [1 2]
[2 4]
```

is not invertible because its determinant is equal to 0

```
det(B) = 1 * 4 - 2 * 2 = 0
```

In fact, B is a singular matrix, which means it does not have an inverse.

**Properties of inverse matrices:**

1. If A and B are invertible matrices, then AB is **invertible** and

```
(AB)^-1 = B^-1 A^-1
```

2. The inverse of a matrix is **unique**.

3. If A is an invertible matrix, then

```
(A^-1)^-1 = A
```

4. If A is an invertible matrix, then

```
(kA)^-1 = (1/k)A^-1
```

for any **nonzero** **scalar** k.

5. If A is an invertible matrix, then the **transpose** of A is also **invertible**

```
(A^T)^-1 = (A^-1)^T
```

For example, consider the matrices

```
A = [1 2]
[3 4]
```

and

```
B = [5 6]
[7 8]
```

We can compute their product AB and its inverse as follows:

```
AB = [15 + 27 16 + 28] = [19 22]
[35 + 47 36 + 48] [43 50]
```

```
det(AB) = 19 * 50 - 22 * 43 = 1
```

and

```
(AB)^-1 = (1/det(AB)) * [ 50 -22] = [ 50 -22]
[-43 19] [-43 19]
```

We can also compute the inverses of A and B separately:

```
det(A) = 14 - 23 = -2, A^-1 = (-1/2) * [ 4 -2] = [ -2 1]
[-3 1] [3/2 -1/2]
```

```
det(B) = 58 - 67 = -2, B^-1 = (-1/2) * [ 8 -6] = [-2 3/2]
[-7 5] [7/2 -5/2]
```

Then, using the property (1) above, we can compute the inverse of AB as follows:

```
(AB)^-1 = B^-1 * A^-1 = [ -2 3/2] * [ -2 1] = [ 50 -22]
[7/2 -5/2] [3/2 -1/2] [-43 19]
```

Therefore, we have verified that the inverse of AB is indeed

```
(AB)^-1 = [50 -22]
[-43 19]
```

**Singular matrices and their implications:**

A matrix A is said to be singular if its determinant det(A) = 0. In this case, A does not have an inverse. Singular matrices have important implications in linear algebra and applications. For example:

- A system of linear equations Ax = b may have no solutions or infinitely many solutions if A is singular.
- The columns of a singular matrix A are linearly dependent, which means that at least one column is a linear combination of the other columns.
- The eigenvalues of a singular matrix A are 0 or can be very small, which can cause instability or numerical issues in computations involving A.

**Applications of determinants and inverse matrices in various fields:**

Determinants and inverse matrices have numerous applications in various fields such as physics, engineering, economics, and computer science. Some examples include:

- In physics, determinants are used to compute the volume, surface area, and other geometrical properties of objects in space and to solve problems in mechanics, electromagnetism, and quantum mechanics.
- In engineering, inverse matrices are used to solve systems of linear equations, to compute the coefficients of polynomial curves and surfaces, and to model and simulate

**Conclusion**

In conclusion, determinants and inverse networks are principal concepts in linear algebra that have broad applications in different areas. Determinants give critical data around the direct change spoken to by a network, whereas converse frameworks permit us to compute the reverse of a lattice and disentangle computations. Understanding these concepts and their properties is pivotal for solving frameworks of straight conditions, modelling and simulation, and computing geometrical properties.

**Key takeaways**

- Determinants are scalar values that can be computed from square matrices, and provide information about the linear transformation represented by the matrix.
- Inverse matrices are matrices that can be multiplied by the original matrix to obtain the identity matrix and can be computed using the determinant and the adjugate of the matrix.
- The properties of determinant and inverse matrices, such as uniqueness and algebraic operations, are useful for simplifying computations and proving results in linear algebra.
- Singular matrices, which have zero determinants, have important implications in linear algebra and applications, such as systems of linear equations and stability.
- Determinants and inverse matrices have numerous applications in various fields such as physics, engineering, economics, and computer science, including solving systems of linear equations, modeling and simulation, and computing geometrical properties.

**Quiz**

**1. Which of the following statements is true about the determinant of a matrix?**

a. The determinant is always positive b. The determinant is always negative c. The determinant is zero if and only if the matrix is singular d. The determinant is zero if and only if the matrix is invertible

**Answer**: c. The determinant is zero if and only if the matrix is singular.

**2. Which of the following is the formula for computing the inverse of a 2x2 matrix A?**

a. A^-1 = 1/det(A) * adj(A) b. A^-1 = 1/det(A) * transpose(A) c. A^-1 = 1/det(A) * inverse(A) d. A^-1 = 1/det(A) * adjugate(A)

**Answer**: d) A^-1 = 1/det(A) * adjugate(A)

**3. Which of the following is true about the product of a matrix A and its inverse A^-1?**

a. The product is always equal to the identity matrix b. The product is always equal to the zero matrix c. The product is only defined if A is invertible d. The product is only defined if A is singular

**Answer**: a. The product is always equal to the identity matrix.

**4. Which of the following statements is true about the adjugate of a matrix A?**

a. The adjugate of A is always equal to the transpose of A b. The adjugate of A is always equal to the inverse of A c. The adjugate of A is only defined if A is invertible d. The adjugate of A is always equal to the determinant of A times the inverse of A

**Answer**: d. The adjugate of A is always equal to the determinant of A times the inverse of A.

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