Maths Olympiad Prep

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Number theory Difficulty 5.6 AIME, harder Prove it Austria

Let xx and yy be integers for which x+y0x + y \neq 0 holds. Determine all pairs (x,y)(x, y) satisfying
x2+y2x+y=10. \frac{x^2 + y^2}{x + y} = 10.

Solution

(x,y){(2,4),(2,6),(0,10),(4,2),(4,12),(6,2),(6,12),(10,0),(10,10),(12,4),(12,6)}. (x, y) \in \{(-2, 4), (-2, 6), (0, 10), (4, -2), (4, 12), (6, -2), (6, 12), (10, 0), (10, 10), (12, 4), (12, 6)\}.

An equivalent form of the given equation is
x2+y2=10x+10y    x210x+y210y=0    (x5)2+(y5)2=50 \begin{align*} x^2 + y^2 &= 10x + 10y \\ \iff x^2 - 10x + y^2 - 10y &= 0 \\ \iff (x - 5)^2 + (y - 5)^2 &= 50 \end{align*}
with x+y0x + y \neq 0. We therefore have to solve the equation a2+b2=50a^2 + b^2 = 50 for integers aa and bb.
As max(a2,b2)50\max(a^2, b^2) \leq 50 we get max(a,b)7\max(|a|, |b|) \leq 7. Furthermore we obtain min(a2,b2)25\min(a^2, b^2) \leq 25 which yields min(a,b)5\min(|a|, |b|) \leq 5. Analyzing the individual cases, we obtain (a,b){(±1,±7),(±7,±1),(±5,±5)}(a, b) \in \{(\pm1, \pm7), (\pm7, \pm1), (\pm5, \pm5)\} as the only possible solutions. If (a,b){(±1,±7),(±7,±1)}(a, b) \in \{(\pm1, \pm7), (\pm7, \pm1)\}, we get that x5=±1x - 5 = \pm1 and y5=±7y - 5 = \pm7 (or xx and yy swapped), which yields x{4,6}x \in \{4, 6\} and y{2,12}y \in \{-2, 12\} (or y{4,6}y \in \{4, 6\} and x{2,12}x \in \{-2, 12\}). The case (a,b)=(±5,±5)(a, b) = (\pm5, \pm5) gives x5=±5x - 5 = \pm5 and y5=±5y - 5 = \pm5 which is equivalent to x{0,10}x \in \{0, 10\} and y{0,10}y \in \{0, 10\}. The pair (x,y)=(0,0)(x, y) = (0, 0) is the only one violating x+y0x + y \neq 0. Therefore, we get the (eleven) different pairs listed in the answer.

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Source: MathNet, licensed CC-BY-4.0. Statement and solution reproduced as published; topic and difficulty added by this site.