A SYCOPHANTIC EQUATION

      ... sorry, Diophantine !

          This is Exercise S:E24.305 from a magazine dear to us. The authors are Răzvan LUPU and Roxana VASILE from CRAIOVA. The exercise is proposed for 7th grade. In translation:

          "Determine the integers $x$ and $y$ that verify the relationship

$$x^2-xy+2x-3y=6."$$

ANSWER CiP

$$(x,y)\in \{(0,-2),\;(-2,-6),\;(-4,-2),\;(-6,-6)\}$$

Solution CiP

               We write the equation successively
$$x^2+3x-x-3+3-xy-3y=6\;\;\Leftrightarrow\;\;x(x+3)-(x+3)+3-y(x+3)=6\;\;\Leftrightarrow$$

$$\Leftrightarrow \;\;(x+3)(x-1-y)=3$$

Because $(x,y)\in \mathbb{Z}\times \mathbb{Z}$, we have the cases indicated on the columns in the table:

$$\begin{array}{c||c|c|c|c}x+3&1&-1&3&-3\\ \hline x-1-y&3&-3&1&-1\\ \hline \hline x&-2&-4&0&-6 \\ \hline y&-6&-2&-2&-6 \end{array}$$

We got the answer.

$\blacksquare$

     Remark CiP We guessed the decomposition by following the procedure of completing squares.

$$x^2-xy+2x-3y=x^2-x(y-2)-3y=x^2-2\cdot x\cdot \frac{y-2}{2}-3y=$$

$$=\left (x-\frac{y-2}{2}\right )^2-\underline{\left ( \frac{y-2}{2} \right )^2}-3y=\left (x-\frac{1}{2}y+1 \right)^2-\frac{1}{4}y^2-2y-1=$$

$$=\left(x-\frac{1}{2}y+1 \right )^2-\frac{1}{4}\cdot (y^2+8y+16)+\underline{4}-1=\left (x-\frac{1}{2}y+1 \right )^2-\frac{1}{4}(y+4)^2+3=$$

$$=\left [ \left (x-\frac{1}{2}y+1\right )-\frac{1}{2}(y+4) \right ]\cdot \left [ \left (x-\frac{1}{2}y+1 \right )+\frac{1}{2}(y+4) \right ]+3=$$

$$=(x-y-1) \cdot (x+3)+3...$$

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