I found a systematic way (c.f. How to solve this quadratic congruence equation) to solve all congruent equations of the form of $ax^2+bx+c=0\pmod{p}$, or to determine that they have no solution.

But I wonder if there is some easy way to find solutions of simple quadratic congruent equations by inspection, for example, for this equation $$x^2+x+47=0\pmod{7}.$$

My textbook gave the solutions $x\equiv 5\pmod{7}$ and $x\equiv 1\pmod{7}$ directly, without any proof. So I think there must be some easier way to inspect the solutions.

Any help will be appreciated.

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    $\begingroup$ Note: $x^2+x+47\equiv x^2-6x+5\pmod 7$ $\endgroup$ – J. W. Tanner May 10 '19 at 14:23
  • $\begingroup$ Also you can note that $x(x+1)=-5=2$ $\endgroup$ – Piquito May 10 '19 at 14:35
  • $\begingroup$ @J.W.Tanner What an excellent observation! Thanks Tanner. $\endgroup$ – Sam Wong May 10 '19 at 20:54
  • $\begingroup$ @Piquito Thanks for your observation too. But I think Tanner's observation is more straightforward :) $\endgroup$ – Sam Wong May 10 '19 at 20:55

The good old quadratic equation works just fine if $p\neq2$. If $a\not\equiv 0\pmod{p}$ and $b^2-4ac$ is a quadratic residue mod $p$, then the solutions to the quadratic congruence $$ax^2+bx+c\equiv0\pmod{p},$$ are precisely $$-\frac{b\pm\sqrt{b^2-4ac}}{2a}.$$

In this particular case we have $p=7$ and $a\equiv b\equiv1\pmod{7}$ and $c\equiv47\equiv5\pmod{7}$. Then $$b^2-4ac\equiv2\equiv3^2\pmod{7},$$ so the congruence has the two solutions $$-\frac{1+3}{2}=5\qquad\text{ and }\qquad-\frac{1-3}{2}=1.$$

On the other hand, if you want to solve it purely by inspection, note that there are only $7$ possible solutions to check. Clearly $x=0$ is not a solution, and plugging in $x=1$ yields the first solution. The product of the solutions is congruent to $47\equiv5\pmod{7}$, so the other solution is $x=5$.

  • $\begingroup$ Thanks Servaes. But I still have problems in understanding your last sentence. Why is that the product of the solutions is congruent to $47\equiv 5\, ($mod$ \, 7)$ ? And why if it is so, the other solution will be $x=5$? Thanks for your elaboration. $\endgroup$ – Sam Wong May 10 '19 at 20:54
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    $\begingroup$ @SamWong: the product of the roots of $x^2+bx+c=0$ is $c$, so $5$ in this case, and the solution of $1\times x\equiv5$ is $x\equiv5$ $\endgroup$ – J. W. Tanner May 10 '19 at 21:10
  • $\begingroup$ @J.W.Tanner Oh I see. Thanks! $\endgroup$ – Sam Wong May 10 '19 at 22:41

Servaes gave a good general method for solving quadratic equations modulo a prime.

I will elaborate on my comment about how in this particular case the answer could be found by inspection.

Adding or subtracting a multiple of $p$ to or from any of the coefficients $a,b,c$

does not materially change the equation $ax^2+bx+c\equiv0\mod p.$

In fact, $x^2+x+47$ begs to be reduced modulo $7$ to $x^2+x+5.$

$x^2+x+5$ doesn't have rational roots, but you could add or subtract any multiple of $p$ to the $x$-coefficient $1$,

yielding polynomials such as $x^2+8x+5, x^2-6x+5,$ etc.,

and $x^2-6x+5$ is easily recognized as $(x-1)(x-5)$.


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