# how to solve linear equations involving modulo?

In one of the programming work I am doing, I encountered a set of linear eqns. with modulo. I am putting it in simple format with only 2 variables,

$$(a_{11}x + a_{12}y) \mod 8 = b_1$$ $$(a_{21}x + a_{22}y) \mod 8 = b_2$$

In case of simple eqns. with out modulo I can use: $AX=B$, $X=A^{-1}B$. But how to handle this modulo? Any clue or pointers will be very helpful.

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You can do Gauss-Jordan elimination performing operations modulo $8$. Just be mindful that "division" is only defined for odd integers (you are really multiplying by the inverse modulo $8$).

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You can apply Cramer's rule if the determinant is odd (so invertible mod 8). For an introduction to linear algebra over commutative rings see Wm. C. Brown: Matrices over commutative rings.

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