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Algebra Difficulty 7.2 National olympiad, round 2 Find the answer

Example 1.27 Given a cubic equation x3+ax2+bx+c=0(a,b,cR)x^{3} + a x^{2} + b x + c = 0 (a, b, c \in \mathbf{R}) with three roots α\alpha, β\beta, γ\gamma whose magnitudes are all no greater than 1, find
1+a+b+cα+β+γ\frac{1+|a|+|b|+|c|}{|\alpha|+|\beta|+|\gamma|}

the minimum value.

A number or a short expression. Spacing and $ signs are ignored.

Solution

Given the problem, let's assume 1αβγ,β=sα,γ=tα1 \geqslant|\alpha| \geqslant|\beta| \geqslant|\gamma|, \beta=s \alpha, \gamma=t \alpha, then 1st1 \geqslant|s| \geqslant|t|, and let

then
θ=α1,u=1+a+b+cα+β+γu=1+a+b+cα+β+γ=1+θ1+s+t+θ2s+t+st+θ3stθ(1+s+t)1+θ3(1+s+t+s+t+st+st)3θ1+θ3(1+s+t+s+t)3θ1+θ33θ=13(12θ+12θ+θ2)12θ12θθ23=232\begin{aligned} \theta= & |\alpha| \leqslant 1, u=\frac{1+|a|+|b|+|c|}{|\alpha|+|\beta|+|\gamma|} \\ u= & \frac{1+|a|+|b|+|c|}{|\alpha|+|\beta|+|\gamma|}= \\ & \frac{1+\theta|1+s+t|+\theta^{2}|s+t+s t|+\theta^{3}|s t|}{\theta(1+|s|+|t|)} \geqslant \\ & \frac{1+\theta^{3}(|1+s+t|+|s+t+s t|+|s t|)}{3 \theta} \geqslant \\ & \frac{1+\theta^{3}(|1+s+t|+|s+t|)}{3 \theta} \geqslant \frac{1+\theta^{3}}{3 \theta}= \\ & \frac{1}{3}\left(\frac{1}{2 \theta}+\frac{1}{2 \theta}+\theta^{2}\right) \geqslant \sqrt[3]{\frac{1}{2 \theta} \cdot \frac{1}{2 \theta} \cdot \theta^{2}}=\frac{\sqrt[3]{2}}{2} \end{aligned}

It is easy to verify that when θ=432,s=1+3i2,t=13i2\theta=\frac{\sqrt[3]{4}}{2}, s=-\frac{-1+\sqrt{3} \mathrm{i}}{2}, t=-\frac{-1-\sqrt{3} \mathrm{i}}{2}, uu reaches its minimum value 232\frac{\sqrt[3]{2}}{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.