# Math Help - Linear Congruence

1. ## Linear Congruence

The following was confusing me, so any help/explanations would be greatly appreciated!

Find all incongruent solutions to the following linear congruence:

21x ≡ 14 (mod 91)

2. Originally Posted by clockingly
The following was confusing me, so any help/explanations would be greatly appreciated!

Find all incongruent solutions to the following linear congruence:

21x ≡ 14 (mod 91)
$\gcd(21,91)=7$ and $7|14$ so there shall be $7$ solutions in each congruence class. By inspection we see that $x\equiv 5$ is one equivalence class which solves this. By theorem it means $5 + \frac{91}{7}t$ for $t=0,1,...,6$ shall all be solutions.

3. Originally Posted by ThePerfectHacker
$\gcd(21,91)=7$ and $7|14$ so there shall be $7$ solutions in each congruence class. By inspection we see that $x\equiv 5$ is one equivalence class which solves this. By theorem it means $5 + \frac{91}{7}t$ for $t=0,1,...,6$ shall all be solutions.
By "incongruent" solutions the question is asking for all distinct residue classes that solve the equation?

-Dan

4. Originally Posted by topsquark
By "incongruent" solutions the question is asking for all distinct residue classes that solve the equation?

-Dan
Yes.

For example $x\equiv 2 (\bmod 4)$ the solutions $x=2,6$ are really the same ("congruent") solutions because they are contained in the same equivalence class (equivalence class mod $4$*)

*)The congruence classes are $\{...,-4,0,4,...\} , \{...,-3,1,5,...\}, \{...,-2,2,6,...\}, \{...,-1,3,7,...\}$. So $x=2,6$ are in the same equivalence class.

5. ## hmm

so if showing the incongruences would it be x=-12=14 (mod13)??