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Algebra Difficulty 6.8 National olympiad Find the answer

Example 6 Find all positive integers n2n \geqslant 2, such that the system of equations
{x12+x22+50=16x1+12x2x22+x32+50=16x2+12x3xn12+xn2+50=16xn1+12xnxn2+x12+50=16xn+12x1\left\{\begin{array}{c} x_{1}^{2}+x_{2}^{2}+50=16 x_{1}+12 x_{2} \\ x_{2}^{2}+x_{3}^{2}+50=16 x_{2}+12 x_{3} \\ \cdots \\ x_{n-1}^{2}+x_{n}^{2}+50=16 x_{n-1}+12 x_{n} \\ x_{n}^{2}+x_{1}^{2}+50=16 x_{n}+12 x_{1} \end{array}\right.

has integer solutions.

A number or a short expression. Fractions can be typed as 3/2, and spacing doesn't matter.

Solution

Solving by rearranging and completing the square, the system of equations transforms to
{(x18)2+(x26)2=50(x28)2+(x36)2=50(xn18)2+(xn6)2=50(xn8)2+(x16)2=50\left\{\begin{array}{c} \left(x_{1}-8\right)^{2}+\left(x_{2}-6\right)^{2}=50 \\ \left(x_{2}-8\right)^{2}+\left(x_{3}-6\right)^{2}=50 \\ \cdots \\ \left(x_{n-1}-8\right)^{2}+\left(x_{n}-6\right)^{2}=50 \\ \left(x_{n}-8\right)^{2}+\left(x_{1}-6\right)^{2}=50 \end{array}\right.

Since 50 can be expressed as the sum of squares of two positive integers in only two ways: 50=12+72=52+5250=1^{2}+7^{2}=5^{2}+5^{2}, it follows from (1) that x26=1,5\left|x_{2}-6\right|=1, 5 or 7, and from (2) that x28=1,5\left|x_{2}-8\right|=1, 5 or 7, thus x2=1,7x_{2}=1, 7 or 13.

Furthermore, for each 1in1 \leqslant i \leqslant n, we have xi=1,7x_{i}=1, 7 or 13. Depending on x1=1,7,13x_{1}=1, 7, 13, we consider three cases.

If x1=1x_{1}=1, then from (1) we know x2=7x_{2}=7, and from (2) we know x3=13x_{3}=13, and so on. Thus, when k1(mod3)k \equiv 1(\bmod 3), xk=1x_{k}=1; when k2(mod3)k \equiv 2(\bmod 3), xk=7x_{k}=7; and when k0(mod3)k \equiv 0(\bmod 3), xk=13x_{k}=13. Therefore, by the second equation, we know that the original system of equations has integer solutions if and only if n+11(mod3)n+1 \equiv 1(\bmod 3), i.e., if and only if 3n3 \mid n, nn meets the requirement.

For the other two cases x1=7x_{1}=7 and x1=13x_{1}=13, similar discussions yield the same condition.
In summary, the nn that satisfies the condition are all multiples of 3.

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Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.