Maths Olympiad Prep

Library / /3 of 5

Algebra Difficulty 4.8 AIME Prove it Japan

Suppose the real numbers x1,x2,x3,x4,x5x_1, x_2, x_3, x_4, x_5 satisfy the following set of five equations:
{x1x2+x1x3+x1x4+x1x5=1x2x1+x2x3+x2x4+x2x5=1x3x1+x3x2+x3x4+x3x5=1x4x1+x4x2+x4x3+x4x5=1x5x1+x5x2+x5x3+x5x4=1 \begin{cases} x_1x_2 + x_1x_3 + x_1x_4 + x_1x_5 = -1 \\ x_2x_1 + x_2x_3 + x_2x_4 + x_2x_5 = -1 \\ x_3x_1 + x_3x_2 + x_3x_4 + x_3x_5 = -1 \\ x_4x_1 + x_4x_2 + x_4x_3 + x_4x_5 = -1 \\ x_5x_1 + x_5x_2 + x_5x_3 + x_5x_4 = -1 \end{cases}
Determine all the possible values that the number x1x_1 can take.

Solution

Suppose the real numbers x1,x2,,x5x_1, x_2, \dots, x_5 satisfy the given set of equations. Let a=x1+x2++x5a = x_1 + x_2 + \dots + x_5. Then, each of the five numbers xj,j=1,2,,5x_j, j = 1, 2, \dots, 5 satisfies the quadratic equation x2ax1=0x^2 - a x - 1 = 0. This equation has 2 distinct real roots a±a2+42\frac{a \pm \sqrt{a^2 + 4}}{2}. Denote one of the roots equaling x1x_1 by α\alpha and the other by β\beta. Then we have αβ=1\alpha \beta = -1. Furthermore, if we assume that exactly kk among the numbers xj,j=1,2,,5x_j, j = 1, 2, \dots, 5 equal α\alpha, and 5k5-k among them equal β\beta, we get a=kα+(5k)β=kα5kαa = k\alpha + (5-k)\beta = k\alpha - \frac{5-k}{\alpha}. Substituting this into the equation α2aα1=0\alpha^2 - a\alpha - 1 = 0, we obtain (k1)α2=4k(k-1)\alpha^2 = 4-k. Since α2>0\alpha^2 > 0, we must have either k=2k=2 or k=3k=3. If we have k=2k=2, then we get α=±2\alpha = \pm\sqrt{2}, while if k=3k=3, we get α=±22\alpha = \pm\frac{\sqrt{2}}{2}. It is easy to check that each of these values of α\alpha and the corresponding values for β\beta satisfy the quadratic equation x2ax1=0x^2 - a x - 1 = 0, so the desired solution for the problem is given by ±2,±22\pm\sqrt{2}, \pm\frac{\sqrt{2}}{2}.

Want a route through all this instead of an archive? The track puts 2,000 problems in a working order, from AMC 10 level to the IMO shortlist.

Source: MathNet, licensed CC-BY-4.0. Statement reproduced verbatim; metadata (topic, difficulty) added by this project.