Sunday, October 27, 2019

modular arithmetic - Solving linear equivalence mod $26$

I am looking to solve for $x$ in this modular arithmetic problem. We haven't learned anything remotely as complex as this in my class so not exactly sure where to start.



$$y \equiv 5x + 25 \pmod{26}$$



Here's what I know so far.



The additive inverse of 25 in mod 26 is 1 since: $1 + 25 \equiv 0 \pmod{26}$. So, I added 1 to both sides of the congruence.




The multiplicative inverse of 5 in mod 26 is −5 since: $5(−5) \equiv −25 \equiv 1 \pmod{26}$. So, I multiplied by -5 on both sides of the congruence.



After doing those steps this is what I have come up with



$$-5 - 5y \equiv ( -25x - 130) \pmod{26} \tag 1$$



Then I add the 5 over to the other side of the equation to get



$$-5y \equiv -25x - 125 \pmod{26} \tag 2$$




Then I divided by a factor of 5 to each of the numbers



$$-y \equiv -5x - 25 \pmod{26} \tag 3$$



Now I feel stuck and like I'm just back at the start... hmm is this even correct? Let me know where I went wrong or what I need to continue doing! Thanks in advance

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