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Algebra Difficulty 6.4 National olympiad Find the answer

37 Let aa, bb, cc all be positive integers greater than 1. Find the minimum possible value of the algebraic expression a+b+c2[a,b]+[b,c]+[c,a]a+b+c\frac{a+b+c}{2}-\frac{[a, b]+[b, c]+[c, a]}{a+b+c}.

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

Solution

37. By symmetry, we may assume abca \geqslant b \geqslant c. Note that when (a,b,c)=(2,2,2),(3,2,2),(3,3,2),(4,2,2)(a, b, c)=(2,2,2),(3,2,2),(3,3,2),(4,2,2), the value of the given algebraic expression AA is 2,32,178,1142, \frac{3}{2}, \frac{17}{8}, \frac{11}{4}, respectively. This indicates that when a+b+c8a+b+c \leqslant 8, A32A \geqslant \frac{3}{2}.

Next, we prove that when a+b+c9a+b+c \geqslant 9, we have A32A \geqslant \frac{3}{2}.
In fact,
A32(a+b+c)22([a,b]+[b,c]+[c,a])3(a+b+c)a2+b2+c2+2(ab[a,b])3(a+b+c)\begin{aligned} A \geqslant \frac{3}{2} & \Leftrightarrow (a+b+c)^{2}-2([a, b]+[b, c]+[c, a]) \geqslant 3(a+b+c) \\ & \Leftrightarrow a^{2}+b^{2}+c^{2}+2 \sum(a b-[a, b]) \geqslant 3(a+b+c) \end{aligned}

Since for positive integers x,yx, y, we have xy[x,y]x y \geqslant [x, y], it suffices to prove:
a2+b2+c23(a+b+c)a^{2}+b^{2}+c^{2} \geqslant 3(a+b+c)

Given a+b+c9a+b+c \geqslant 9, to prove (1) holds, it suffices to prove:
a2+b2+c213(a+b+c)23(a2+b2+c2)(a+b+c)22(a2+b2+c2)2(ab+bc+ca)0(ab)2+(bc)2+(ca)20\begin{aligned} & a^{2}+b^{2}+c^{2} \geqslant \frac{1}{3}(a+b+c)^{2} \\ \Leftrightarrow & 3\left(a^{2}+b^{2}+c^{2}\right) \geqslant (a+b+c)^{2} \\ \Leftrightarrow & 2\left(a^{2}+b^{2}+c^{2}\right)-2(a b+b c+c a) \geqslant 0 \\ \Leftrightarrow & (a-b)^{2}+(b-c)^{2}+(c-a)^{2} \geqslant 0 \end{aligned}

The last inequality is obviously true.
Therefore, the minimum value of the given algebraic expression is 32\frac{3}{2}.

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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.