Consider the following system of 10 equations in 10 real variables v1,…,v10: vi=1+v12+v22+⋯+v1026vi2(i=1,…,10). Find all 10-tuples (v1,v2,…,v10) that are solutions of this system.
Solution
For a particular solution (v1,v2,…,v10), let s=v12+v22+⋯+v102. Then vi=1+s6vi2⇒6vi2−svi+s=0. Let a and b be the roots of the quadratic 6x2−sx+s=0, so for each i, vi=a or vi=b. We also have ab=s/6 (by Vieta's formula, for example). If all the vi are equal, then vi=1+106=58 for all i. Otherwise, let 5+k of the vi be a, and let 5−k of the vi be b, where 0<k≤4. Then by the AM-GM inequality, 6ab=s=(5+k)a2+(5−k)b2≥2ab25−k2. From the given equations, vi≥1 for all i, so a and b are positive. Then 25−k2≤3⇒25−k2≤9⇒k2≥16⇒k=4. Hence, 6ab=9a2+b2⇒(b−3a)2=0⇒b=3a. Adding all given ten equations, we get v1+v2+⋯+v10=16. But v1+v2+⋯+v10=9a+b=12a, so a=16/12=4/3 and b=4. Therefore, the solutions are (8/5,8/5,…,8/5) and all ten permutations of (4/3,4/3,…,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.