AlgebraDifficulty 6.8National olympiadFind the answer
Example 1 Let real numbers x1,x2,⋯,x1,⋯77 satisfy the following two conditions: (1) −31⩽xi⩽3(i=1,2,⋯,1997); (2) x1+x2+⋯+x1997=−3183.
Try to find the maximum value of x112+x212+⋯+x199712. (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,⋯,x1997 that satisfies the given conditions. If there are such xi and xj: 3>xi⩾xj>−31, then let m=21(xi+xj),h0=21(xi−xj)=xi−m=m−xj. We observe that (m+h)12+(m−h)12=20⩽k=2l⩽12∑C12km12−khk
increases as h>0 increases. We agree to take h=min{3−m,m−(−31)}, and replace xi and xj with x′i=m+h,x′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 (−31,3), and the other variables are either −31 or 3.
Suppose u variables take the value −31,v variables take the value 3,w(=0 or 1) variables take a value in (−31,3) (if there is one, denote this value as t). Thus, {u+v+w=1997−31u+3v+tw=−3183
From this, we get 4v+(3t+1)w=1043
Since (3t+1)w=1043−4v is an integer, and 0⩽1043−4v<4, (3t+1)w is the remainder when 1043 is divided by 4. According to this, we find v=260,t=32,u=1736
According to the above discussion, the maximum value of x112+x212+⋯+x199712 is (−31)12u+(3)12v+t12=7291736+4096+729×260=8+189540=189548
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