AlgebraDifficulty 7.2National olympiad, round 2Find the answer
Example 1.27 Given a cubic equation x3+ax2+bx+c=0(a,b,c∈R) with three roots α, β, γ whose magnitudes are all no greater than 1, find ∣α∣+∣β∣+∣γ∣1+∣a∣+∣b∣+∣c∣
the minimum value.
A number or a short expression. Spacing and $ signs are ignored.
Solution
Given the problem, let's assume 1⩾∣α∣⩾∣β∣⩾∣γ∣,β=sα,γ=tα, then 1⩾∣s∣⩾∣t∣, and let
then θ=u=∣α∣⩽1,u=∣α∣+∣β∣+∣γ∣1+∣a∣+∣b∣+∣c∣∣α∣+∣β∣+∣γ∣1+∣a∣+∣b∣+∣c∣=θ(1+∣s∣+∣t∣)1+θ∣1+s+t∣+θ2∣s+t+st∣+θ3∣st∣⩾3θ1+θ3(∣1+s+t∣+∣s+t+st∣+∣st∣)⩾3θ1+θ3(∣1+s+t∣+∣s+t∣)⩾3θ1+θ3=31(2θ1+2θ1+θ2)⩾32θ1⋅2θ1⋅θ2=232
It is easy to verify that when θ=234,s=−2−1+3i,t=−2−1−3i, u reaches its minimum value 232.
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Source: NuminaMath-1.5,
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