Solving Congruences: Proving a=b

  • Thread starter ninjagod123
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In summary, if a \equiv b mod p for all primes p, then a = b because the only way for (a - b) to be divisible by all primes is if it is equal to zero. This means that every nonzero integer can only be divisible by a finite number of primes. Therefore, if a and b are not equal, there will always be a prime that cannot divide (a - b), making the congruence false. Alternatively, if b > a, we can use b - a instead to achieve the same result.
  • #1
ninjagod123
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Show that if a [tex]\equiv[/tex] b mod p for all primes p, then a = b.
 
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  • #2
Well, a - b must be divisible by all primes p. What is the only way for this to happen?
 
  • #3
JSuarez said:
Well, a - b must be divisible by all primes p. What is the only way for this to happen?

Oh hmmm...

The only way is if (a - b) is zero. How would I formally write this up? I guess a - b can't be the product of all primes?
 
  • #4
Every nonzero integer can only be divisible by a finite number of primes.
 
  • #5
ninjagod123 said:
I guess a - b can't be the product of all primes?

In a sense, that's what 0 is. It's the "infinity" of the divisibility relation.
 
  • #6
If [itex]a> b[/itex] then a- b is a positive number. Since there are an infinite number of primes, there exist a prime, p> a- b. Then p cannot divide a- b so [itex]a\ne b (mod p)[/itex].

If [itex]b> a[/itex] just use b- a instead of a- b.
 

1. What is a congruence?

A congruence is a mathematical concept that describes the relationship between two numbers or shapes that have the same size, shape, and orientation. In other words, two objects are congruent if they are exactly the same, but may be in different positions or orientations.

2. How is a congruence different from an equality?

While equality means that two quantities are exactly the same, congruence allows for differences in position or orientation. For example, two triangles can be congruent if they have the same sides and angles, but they can be in different positions on a plane.

3. What is the process for solving congruences?

The process for solving congruences depends on the type of congruence. For proving a=b, the first step is to use the given information to identify any known congruent sides or angles. Then, use properties of congruent triangles, such as the Side-Side-Side (SSS) or Angle-Side-Angle (ASA) congruence criteria, to prove that the remaining sides and angles are congruent.

4. How can I check if my solution to a congruence is correct?

To check if your solution to a congruence is correct, you can use substitution. Simply plug in the values for the congruent sides and angles into the original congruence statement. If the values are equal, then your solution is correct.

5. Can there be more than one solution to a congruence?

Yes, there can be more than one solution to a congruence. This is because there may be multiple ways to prove that two objects are congruent. However, it is important to use all given information and follow the appropriate congruence criteria to ensure that your solution is correct.

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