Are There Multiple Solutions to this System of Linear Congruences?

In summary, the given system of equations is solved by eliminating one variable and using the modulo property. The resulting values for x and y are (17,8), but there should not be more solutions since there are only two unknowns and two linear equations. It is recommended to check the solution by substituting it back into the original equations and considering the system in different modulos.
  • #1
oliver$
6
0
the system:

24x + 11y= 4 (mod 35)
5x + 7y= -13 (mod 35)

is solved to get:

-113y= 111 (mod 35)
113x= 171 (mod 35)

which gives: (17,8).

should there not be more solutions?
 
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  • #2
No, with two unknowns, x and y, we need exactly two linear equations to find the unique solution. Now, if x or y was quadratic, it would be different.

You can see from your own work that, we use one of the equations to eliminate one of the terms. Thus what remains is all in one unknown.

However, since it is a modulo equation, you can add or subtract any multiple of 35 to x or y.
 
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  • #3
You might want to check your solution by substituting it back into the original equations.
 
  • #4
Ask yourself two questions:

(1) Why do you think there should be more solutions?
(2) What things do you know about solutions to systems of linear equations?

When answering these questions, it might help to consider things mod 7 and mod 5, so that you're working over a field (and thus most of what you learned in linear algebra is applicable)
 
  • #5
Oliver$: should there not be more solutions?

Unfortunately "Yes," since there is a mistake in your value for y.

It might be easier as Hurkyl suggests to work with modulo 5 and modulo 7.
 
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  • #6
ok, upon doing it (mod 7) and (mod 5), i got (3,4) (mod 7) and (1,2) (mod 7). does that sit well? or should i now apply the CRT?
 
  • #7
ascheras said:
ok, upon doing it (mod 7) and (mod 5), i got (3,4) (mod 7) and (1,2) (mod 7). does that sit well? or should i now apply the CRT?

You can use the CRT after, but you might want to check your (1,2) answer mod 5 (I assume that's your mod 5 solution).
 
  • #8
sorry, i meant (2,1).
 

1. What is a system of linear congruences?

A system of linear congruences is a set of equations that involve modular arithmetic, where the unknown variables are integers and the equations are congruent modulo some integer. In simpler terms, it is a collection of equations with the goal of finding integer solutions that satisfy all the equations simultaneously.

2. What is the Chinese Remainder Theorem and how does it relate to systems of linear congruences?

The Chinese Remainder Theorem is a theorem in number theory that provides a method for solving a system of linear congruences. It states that if the moduli of the congruences are pairwise relatively prime, then there exists a unique solution to the system. This theorem is often used to efficiently solve systems of congruences in cryptography and other areas of mathematics.

3. Can a system of linear congruences have more than one solution?

Yes, a system of linear congruences can have more than one solution. In fact, there can be infinitely many solutions depending on the moduli and equations involved. However, the Chinese Remainder Theorem guarantees that if the moduli are pairwise relatively prime, then there exists a unique solution.

4. How is a system of linear congruences solved?

The most common method for solving a system of linear congruences is through the Chinese Remainder Theorem. This involves breaking down the system into smaller, simpler congruences and using the theorem to find the unique solution. Other methods include substitution and elimination, similar to solving systems of linear equations.

5. What are some real-world applications of systems of linear congruences?

Systems of linear congruences have various applications in cryptography, such as in the RSA encryption algorithm. They are also used in creating schedules and timetables, as well as in solving problems in number theory and combinatorics. Additionally, they can be applied in solving puzzles and games, such as Sudoku and KenKen.

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