On the Quadratic Formula Modulo N

Let $a, b, c,$ and $n$ be integers, with $a$ nonzero and $n$ at least two. Necessary and sufficient conditions on these parameters are derived which guarantee that all solutions of the congruence $ax^2+bx+c \equiv 0\ \textrm{mod}\ n$ are given precisely by the solutions of $2ax\equiv -b+s \ \textrm{mod}\ n,$ where $s$ varies over all solutions of $x^2\equiv b^2-4ac \ \textrm{mod}\ n.$ Corollaries of this result are deduced for prime-power moduli and some illustrative examples are also presented.

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