Congruence problem

mattmns

Hello I have the following question.
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Solve the following congruences simultaneously:

[tex]2x + 3y \equiv 2 mod 631[/tex]
[tex]3x + 2y \equiv 3 mod 631[/tex]
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I first tried adding and got [tex]5x + 5y \equiv 5 mod 631[/tex], but I was then stuck, so I tried the old multiplication, which looked worse as: [tex]6x^2 + 13xy + 6y^2 \equiv 6 mod 631[/tex]

Any ideas? I am guessing that I need to go somewhere with the addition one, but I can't see where. The instructor had a hint of using the fact that 631 is prime, but I can't see anything from that. Thanks.

Hmm, just got an idea, [tex]5(x + y) \equiv 5 mod 631[/tex], then find inverse of 5 and multiply it through to find x+y = something mod 631. I did this and got:

[tex]x + y \equiv 5*79380 mod 631[/tex]
which is
[tex]x + y \equiv 1 mod 631[/tex]
Now where can I go from here? Any ideas?

Gokul43201

Try solving them like regular simultaneous equations.

Eliminate x from the equations. You have a congruence relation for y. Now plug this back in and find a separate congruence relation for x.

mattmns

How would I do that without canceling the y's? In my mind I would have to multiply the top by 3 and bottom by 2, but when I added, I would get 0 = 0 mod 631.

Gokul43201

No, you won't !


[tex]3x + 2y \equiv 3 ~~(mod~ 631)~~~~--(1)[/tex]
[tex]2x + 3y \equiv 2 ~~(mod~ 631)~~~~--(2)[/tex]

Let's do (1)*2 - (2)*3 ; this is legal since the modulo is the same for both congruences.


[tex]6x + 4y \equiv 6 ~~(mod~ 631)~~~~--(3)[/tex]
[tex]6x + 9y \equiv 6 ~~(mod~ 631)~~~~--(4)[/tex]

Subtracting gives : [tex]5y \equiv 0~~ (mod~631) [/tex]

What does this tell you about 'y' ? Plug that back in and figure out what 'x' should be.

mattmns

Wow, I am retarded !!!! Thanks for pointing that out Gokul. Hmm, 3 times 3 is 9, and not 6, interesting! Thanks Gokul!

HallsofIvy

[tex]2x + 3y \equiv 2 mod 631[/tex]
[tex]3x + 2y \equiv 3 mod 631[/tex]

Did you notice that the right hand side is the same as the coefficients of x on the left? Hmm, suppose x= 1?

Gokul43201

That's the clever way to do it. Us mortals have to turn the cranks....