Maths Olympiad Prep

Library / /7 of 19

Algebra Difficulty 6.0 AIME, harder Prove it Canada

Consider the following system of 10 equations in 10 real variables v1,,v10v_1, \dots, v_{10}:
vi=1+6vi2v12+v22++v102(i=1,,10). v_i = 1 + \frac{6 v_i^2}{v_1^2 + v_2^2 + \dots + v_{10}^2} \quad (i = 1, \dots, 10).
Find all 10-tuples (v1,v2,,v10)(v_1, v_2, \dots, v_{10}) that are solutions of this system.

Solution

For a particular solution (v1,v2,,v10)(v_1, v_2, \dots, v_{10}), let s=v12+v22++v102s = v_1^2 + v_2^2 + \dots + v_{10}^2.
Then
vi=1+6vi2s6vi2svi+s=0. v_i = 1 + \frac{6v_i^2}{s} \quad \Rightarrow \quad 6v_i^2 - sv_i + s = 0.
Let aa and bb be the roots of the quadratic 6x2sx+s=06x^2 - sx + s = 0, so for each ii, vi=av_i = a or vi=bv_i = b. We also have ab=s/6ab = s/6 (by Vieta's formula, for example).
If all the viv_i are equal, then
vi=1+610=85 v_i = 1 + \frac{6}{10} = \frac{8}{5}
for all ii. Otherwise, let 5+k5+k of the viv_i be aa, and let 5k5-k of the viv_i be bb, where 0<k40 < k \le 4. Then by the AM-GM inequality,
6ab=s=(5+k)a2+(5k)b22ab25k2. 6ab = s = (5 + k)a^2 + (5 - k)b^2 \ge 2ab\sqrt{25 - k^2}.
From the given equations, vi1v_i \ge 1 for all ii, so aa and bb are positive.
Then 25k2325k29k216k=4\sqrt{25 - k^2} \le 3 \Rightarrow 25 - k^2 \le 9 \Rightarrow k^2 \ge 16 \Rightarrow k = 4. Hence,
6ab=9a2+b2(b3a)2=0b=3a6ab = 9a^2 + b^2 \Rightarrow (b - 3a)^2 = 0 \Rightarrow b = 3a.
Adding all given ten equations, we get
v1+v2++v10=16. v_1 + v_2 + \dots + v_{10} = 16.
But v1+v2++v10=9a+b=12av_1 + v_2 + \dots + v_{10} = 9a + b = 12a, so a=16/12=4/3a = 16/12 = 4/3 and b=4b = 4. Therefore, the solutions are (8/5,8/5,,8/5)(8/5, 8/5, \dots, 8/5) and all ten permutations of (4/3,4/3,,4/3,4)(4/3, 4/3, \dots, 4/3, 4).

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.