Thus, a+b is a multiple of 24.

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In summary, the conversation discusses proving that if a and b are positive integers such that ab is one less than a multiple of 24, then a+b is a multiple of 24. The conversation includes multiple attempts at a solution, including using the cancellation rule and finding possible pairs of a and b. Finally, the conversation concludes with a proof using the fact that for a to be relatively prime to 24, its inverse mod 24 must exist.
  • #1
ehrenfest
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[SOLVED] larson 3.2.16c

Homework Statement


Let x be an integer one less than a multiple of 24. Prove that if a and b are positive integers such that ab=x, then a+b is a multiple of 24.

Homework Equations


The Attempt at a Solution


ab=24n-1 implies that ab is congruent to 2 mod 3 and congruent to 7 mod 8

ab=(a+1)(b+1) -ab-1 = (a+1)(b+1)-24n so ab is congruent to (a+1)(b+1) mod 3 and mod 8

I checked this for n up in {1,...,9}. This is obviously true when 24n-1 is prime. The only n I found in that set for which it is not prime are n=4 and n=9 and it works in both cases.

n=4 gives 95=5*19
n=9 gives 215=5*43
 
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  • #2
There are only 4 ways to factor a*b=23 mod 24. You could just find them all. Notice if a*b=23 mod 24 then both a and b are relatively prime to 24. That narrows the search down quite a bit.
 
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  • #3
Dick said:
There are only 4 ways to factor a*b=23 mod 24. You could just find them all. Notice if a*b=23 mod 24 then both a and b are relatively prime to 24. That narrows the search down quite a bit.

Umm... there are C(24,2)+24= 12*23+24= 300 possibilities for a and b and we only eliminated about 50 of them.

EDIT: this is a ridiculous overestimate
 
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  • #4
The numbers relatively prime to 24 are 1, 5, 7, 11, 13, 17, 19, and 23. Notice then that (WLOG), the possible pairs (a, b) satisfying the congruence are (1, 23), (5, 19), (7, 17), and (11, 13), each of which sum to 24.
 
  • #5
Did you find those possible pairs by multiplying mod 24 every possible pair in {1,5,7,11,13,17,19,23} ?
 
  • #6
I applied the cancellation rule: if gcd(c, m) = 1 and ca = cb (mod m), then a = b (mod m). So, for example, if 5a = 5b (mod 24), then a = b (mod 24). Hence there is only one pair (5, 19) involving a 5, and you don't have to try (5, 1), (5, 5), (5, 7), etc.
 
  • #7
AHA.
So if you find (5,something) that works you know (5, something else) will not work. And you wisely selected 19 to try first because if the result is true, then (5,19) is the only one that can work.
 
  • #8
Right. :)
 
  • #9
You know, thinking about this, there is a way to do this without finding all of the pairs. a*b=23 mod 24 is the same as a*b=(-1). So b=(-1/a) (mod 24, of course!). So a+b=0=a-1/a -> 0=a^2-1=(a+1)*(a-1). Since a is relatively prime to 24, it's not divisible by 2 or 3. If a is not divisible by 2 or 3, then (a+1)*(a-1) is divisible by 24.
 
  • #10
Does (1/a) mod 24 mean the inverse of a mod 24? That is, is (1/a) the unique number x such that x*a=1 mod 24? 1/a mod 24 would not be defined if a and 24 were not relatively prime, correct?

Also, I think the arrow in your proof is pointing the wrong way. We are trying to prove that a+b=0 mod 24.
 
  • #11
Sure a is relatively prime to 24, 1/a is it's inverse mod 24. Change the arrow to a <-> if you like.
 

FAQ: Thus, a+b is a multiple of 24.

What does it mean for a+b to be a multiple of 24?

When a number is a multiple of another number, it means that the first number can be divided evenly by the second number, with no remainder. In this case, it means that a+b can be divided evenly by 24, with no remainder.

How do you know if a+b is a multiple of 24?

To determine if a+b is a multiple of 24, you can use the modulus operator, represented by the % symbol, in a mathematical equation. If the result is 0, then a+b is a multiple of 24. For example, if (a+b)%24 = 0, then a+b is a multiple of 24.

Can a and b be any numbers for a+b to be a multiple of 24?

Yes, a and b can be any numbers as long as their sum is a multiple of 24. This means that a and b can be positive or negative integers, fractions, or decimals.

What are some examples of numbers where a+b is a multiple of 24?

Some examples of numbers where a+b is a multiple of 24 are 24, 48, 72, 96, etc. In general, any two numbers that are both multiples of 24 will result in a sum that is also a multiple of 24.

Is a+b always a multiple of 24 if a and b are both multiples of 24?

Yes, if a and b are both multiples of 24, then their sum, a+b, will also be a multiple of 24. This is because when we add two multiples of 24, the result will always be a multiple of 24.

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