Understanding Linear Congruences

Oct 13, 2025 by wbrum.org 48 views

Solving for a and b in a Linear Congruence

In this article, we will delve into the world of linear congruences and explore a specific problem involving the variables a and b. The given equation is a(3x - 10) ≡ 21x + 2b, and our goal is to determine the values of a and b that satisfy this congruence.

Understanding Linear Congruences

A linear congruence is an equation of the form ax ≡ b (mod n), where a, b, and n are integers. In this case, we have a(3x - 10) ≡ 21x + 2b, which can be rewritten as a(3x - 10) - 21x ≡ 2b. Our objective is to find the values of a and b that make this congruence true for all values of x.

Breaking Down the Congruence

To solve for a and b, we need to manipulate the given congruence and isolate the variables. Let's start by expanding the left-hand side of the equation:

a(3x - 10) - 21x ≡ 2b

Expanding the left-hand side, we get:

3ax - 10a - 21x ≡ 2b

Now, let's combine like terms:

(3a - 21)x - 10a ≡ 2b

Isolating the Variables

Our goal is to isolate the variables a and b. To do this, we need to get rid of the x term on the left-hand side. We can do this by multiplying both sides of the equation by the modular inverse of (3a - 21), which is denoted as (3a - 21)^-1.

Multiplying both sides by (3a - 21)^-1, we get:

(3a - 21)^-1((3a - 21)x - 10a) ≡ (3a - 21)^-1(2b)

Simplifying the left-hand side, we get:

x - (3a - 21)^-1(10a) ≡ (3a - 21)^-1(2b)

Finding the Values of a and b

Now that we have isolated the variables, we can find the values of a and b. To do this, we need to find the values of x that satisfy the congruence.

Let's assume that x = 0. Substituting x = 0 into the congruence, we get:

-(3a - 21)^-1(10a) ≡ (3a - 21)^-1(2b)

Simplifying the left-hand side, we get:

-10a(3a - 21)^-1 ≡ 2b(3a - 21)^-1

Multiplying both sides by (3a - 21), we get:

-10a ≡ 2b

Solving for a and b

Now that we have a simplified equation, we can solve for a and b. To do this, we need to find the values of a and b that satisfy the equation.

Let's assume that a = 7. Substituting a = 7 into the equation, we get:

-70 ≡ 2b

Simplifying the left-hand side, we get:

-35 ≡ b

In this article, we have solved for the values of a and b in the linear congruence a(3x - 10) ≡ 21x + 2b. We have manipulated the congruence and isolated the variables, and have found the values of a and b that satisfy the congruence.

The final answer is a = 7 and b = -35.

[1] "Linear Congruences" by Wolfram MathWorld
[2] "Congruences" by Math Open Reference

In our previous article, we solved for the values of a and b in the linear congruence a(3x - 10) ≡ 21x + 2b. In this article, we will answer some common questions related to solving linear congruences.

Q: What is a linear congruence?

A: A linear congruence is an equation of the form ax ≡ b (mod n), where a, b, and n are integers.

Q: How do I solve a linear congruence?

A: To solve a linear congruence, you need to manipulate the equation and isolate the variable x. You can use various techniques such as multiplying both sides by the modular inverse of a, or using the properties of modular arithmetic.

Q: What is the modular inverse of a?

A: The modular inverse of a is an integer x such that ax ≡ 1 (mod n). In other words, when you multiply a and x together, the result is congruent to 1 modulo n.

Q: How do I find the modular inverse of a?

A: To find the modular inverse of a, you can use the Extended Euclidean Algorithm or the modular inverse formula.

Q: What is the Extended Euclidean Algorithm?

A: The Extended Euclidean Algorithm is a method for finding the greatest common divisor (gcd) of two integers a and b, and also for finding the coefficients of Bézout's identity, which are the modular inverse of a and the modular inverse of b.

Q: How do I use the Extended Euclidean Algorithm to find the modular inverse of a?

A: To use the Extended Euclidean Algorithm to find the modular inverse of a, you need to follow these steps:

Find the gcd of a and n using the Euclidean Algorithm.
If the gcd is 1, then the modular inverse of a exists and can be found using the Extended Euclidean Algorithm.
Use the Extended Euclidean Algorithm to find the coefficients of Bézout's identity, which are the modular inverse of a and the modular inverse of n.

Q: What is Bézout's identity?

A: Bézout's identity is a statement that says that for any two integers a and b, there exist integers x and y such that ax + by = gcd(a, b).

Q: How do I use Bézout's identity to find the modular inverse of a?

A: To use Bézout's identity to find the modular inverse of a, you need to follow these steps:

Find the gcd of a and n using the Euclidean Algorithm.
If the gcd is 1, then the modular inverse of a exists and can be found using Bézout's identity.
Use Bézout's identity to find the coefficients x and y, which are the modular inverse of a and the modular inverse of n.

Q: What are some common mistakes to avoid when solving linear congruences?

A: Some common mistakes to avoid when solving linear congruences include:

Not checking if the modular inverse of a exists before using it.
Not using the correct method for finding the modular inverse of a.
Not checking if the solution is unique before presenting it as the final answer.

In this article, we have answered some common questions related to solving linear congruences. We have covered topics such as the definition of a linear congruence, how to solve a linear congruence, and how to find the modular inverse of a. We have also discussed some common mistakes to avoid when solving linear congruences.

The final answer is that solving linear congruences requires a good understanding of modular arithmetic and the properties of linear congruences. It also requires the ability to manipulate equations and isolate variables.