Modular Arithmetic Homework: 1/2*(x-4)(x-5) = 4(x-4)(x-5) (mod 7)

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Homework Help Overview

The discussion revolves around a modular arithmetic problem involving the equation 1/2*(x-4)(x-5) = 4(x-4)(x-5) (mod 7). Participants are exploring the validity of operations involving rational coefficients in modular arithmetic.

Discussion Character

  • Conceptual clarification, Assumption checking

Approaches and Questions Raised

  • Participants are questioning how to handle the rational coefficient of 1/2 in the context of modular arithmetic. There is discussion about finding the multiplicative inverse of integers within the modulus and how this applies to rational numbers. Some participants suggest that since the integers mod 7 form a field, division by nonzero numbers is valid.

Discussion Status

Some participants have provided insights into the concept of multiplicative inverses in modular arithmetic, specifically noting that 1/2 can be represented as 4 (mod 7). Others are exploring similar operations with different denominators, indicating a productive exchange of ideas regarding the manipulation of rational numbers in this context.

Contextual Notes

There is a mention of potential complications when the denominator exceeds the modulus, prompting further inquiry into how to approach such cases. The discussion reflects varying levels of understanding regarding the application of modular arithmetic rules to rational coefficients.

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Homework Statement



Hi everyone, I have a problem in the following modular arithmetic operation

1/2*(x-4)(x-5) = 4(x-4)(x-5) (mod 7) ("=" means congruent in this expression)

Homework Equations





The Attempt at a Solution



I am completely lost on how the operation is valid. If the coefficient is an integer between 0 to 6 then I can easily find its inverse using euclid extended algorithm but the problem here is that the coefficient is 1/2, a rational number. Any help on this problem will be greatly appreciated!
 
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Falken_47 said:

Homework Statement



Hi everyone, I have a problem in the following modular arithmetic operation

1/2*(x-4)(x-5) = 4(x-4)(x-5) (mod 7) ("=" means congruent in this expression)

Homework Equations


The Attempt at a Solution



I am completely lost on how the operation is valid. If the coefficient is an integer between 0 to 6 then I can easily find its inverse using euclid extended algorithm but the problem here is that the coefficient is 1/2, a rational number. Any help on this problem will be greatly appreciated!

4 x 2 = 1 (mod 7)

So 1/2 = 4 (mod 7)

The integers mod 7 are a field so you can always divide by any nonzero number.

Another way to think of it is that 1/2 is 1 * 2-1. What is the multiplicative inverse of 2? It's 4. So 1/2 = 4 (mod 7).
 
1/2= 4 (mod 7) because 4(2)= 8= 1 (mod 7) and that is, after all, the definition of "multiplicative inverse".
 
Oh alright i think I'm starting to get it. So if the question is 1/3*(x-4)(x-5) it is congruent to 5(x-4)(x-5) (mod 7) because 3 * 5 = 1 mod 7?

And what if the denominator is larger than the mod, say 1/30*(x-4)(x-5), should it be:
30^-1 * (x-4)(x-5)
and 30* 4 = 1 mod 7
therefore 1/30*(x-4)(x-5) = 4 * (x-4)(x-5) mod 7 ?
 

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