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Algebra Difficulty 6.8 National olympiad Find the answer

Example 1 Let real numbers x1,x2,,x1,77x_{1}, x_{2}, \cdots, x_{1}, \cdots 77 satisfy the following two conditions:
(1) 13xi3(i=1,2,,1997)-\frac{1}{\sqrt{3}} \leqslant x_{i} \leqslant \sqrt{3}(i=1,2, \cdots, 1997);
(2) x1+x2++x1997=3183x_{1}+x_{2}+\cdots+x_{1997}=-318 \sqrt{3}.

Try to find the maximum value of x112+x212++x199712x_{1}^{12}+x_{2}^{12}+\cdots+x_{1997}^{12}. (1997 China Mathematical Olympiad Problem)

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

Solution

Solve for any set of x1,x2,,x1997x_{1}, x_{2}, \cdots, x_{1997} that satisfies the given conditions. If there are such xix_{i} and xjx_{j}: 3>xixj>13\sqrt{3}>x_{i} \geqslant x_{j}>-\frac{1}{\sqrt{3}}, then let m=12(xi+xj),h0=12(xixj)=xim=mxjm=\frac{1}{2}\left(x_{i}+x_{j}\right), h_{0}=\frac{1}{2}\left(x_{i}-x_{j}\right)=x_{i}-m=m-x_{j}. We observe that
(m+h)12+(mh)12=20k=2l12C12km12khk(m+h)^{12}+(m-h)^{12}=2 \sum_{0 \leqslant k=2 l \leqslant 12} \mathrm{C}_{12}^{k} m^{12-k} h^{k}

increases as h>0h>0 increases. We agree to take h=min{3m,m(13)}h=\min \left\{\sqrt{3}-m, m-\left(-\frac{1}{\sqrt{3}}\right)\right\}, and replace xix_{i} and xjx_{j} with xi=m+h,xj=mhx^{\prime}{ }_{i}=m+h, x^{\prime}{ }_{j}=m-h. The sum of the elements remains unchanged, and the sum of the 12th powers of the elements increases. Therefore, the maximum value of the sum of the 12th powers can only be achieved in the following scenario: at most one variable takes a value in (13,3)\left(-\frac{1}{\sqrt{3}}, \sqrt{3}\right), and the other variables are either 13-\frac{1}{\sqrt{3}} or 3\sqrt{3}.

Suppose uu variables take the value 13,v-\frac{1}{\sqrt{3}}, v variables take the value 3,w(=0\sqrt{3}, w(=0 or 1)) variables take a value in (13,3)\left(-\frac{1}{\sqrt{3}}, \sqrt{3}\right) (if there is one, denote this value as tt). Thus,
{u+v+w=199713u+3v+tw=3183\left\{\begin{array}{l} u+v+w=1997 \\ -\frac{1}{\sqrt{3}} u+\sqrt{3} v+t w=-318 \sqrt{3} \end{array}\right.

From this, we get
4v+(3t+1)w=10434 v+(\sqrt{3} t+1) w=1043

Since (3t+1)w=10434v(\sqrt{3} t+1) w=1043-4 v is an integer, and 010434v<40 \leqslant 1043-4 v<4, (3t+1)w(\sqrt{3} t+1) w is the remainder when 1043 is divided by 4. According to this, we find
v=260,t=23,u=1736v=260, t=\frac{2}{\sqrt{3}}, u=1736

According to the above discussion, the maximum value of x112+x212++x199712x_{1}^{12}+x_{2}^{12}+\cdots+x_{1997}^{12} is
(13)12u+(3)12v+t12=1736+4096729+729×260=8+189540=189548\begin{array}{l} \left(-\frac{1}{\sqrt{3}}\right)^{12} u+(\sqrt{3})^{12} v+t^{12}=\frac{1736+4096}{729}+729 \times 260= \\ 8+189540=189548 \end{array}

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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.