Again Minimum

Algebra Level 5

c y c a , b , c [ a 2 + b 2 a b ] \displaystyle \sum_{cyc}^{a,b,c}\left[\frac{a^{2}+b^{2}}{ab}\right]

If a , b , c a,b,c be the roots of x 3 + ( ω 4 + 4 ω 2 + 1 ) x = x 2 + ω 2 x^{3}+(\omega^{4}+4\omega^{2}+1)x=x^{2}+\omega^{2} , where ω \omega is some real number, then find the minimum value of the above expression.


The answer is 3.

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1 solution

Applying the Viete's theorem, we have { a + b + c = 1 a b + b c + c a = ω 4 + 4 ω 2 + 1 a b c = ω 2 \left\{\begin{array}{l}a+b+c=1\\ab+bc+ca=\omega^4+4\omega^2+1\\abc=\omega^2\end{array}\right.

We have:

c y c a 2 + b 2 a b = a 2 c + b 2 c + b 2 a + c 2 a + c 2 b + a 2 b a b c \displaystyle \sum_{cyc} \dfrac{a^2+b^2}{ab}=\dfrac{a^2c+b^2c+b^2a+c^2a+c^2b+a^2b}{abc}

= ( a + b + c ) ( a b + b c + c a ) 3 a b c a b c \qquad\qquad\quad\;=\dfrac{(a+b+c)(ab+bc+ca)-3abc}{abc}

= ω 4 + 4 ω 2 + 1 3 ω 2 ω 2 \qquad\qquad\quad\;=\dfrac{\omega^4+4\omega^2+1-3\omega^2}{\omega^2}

= ω 2 + 1 ω 2 + 1 \qquad\qquad\quad\;=\omega^2+\dfrac{1}{\omega^2}+1

2 ω 2 . 1 ω 2 + 1 = 3 \qquad\qquad\quad\;\ge2\sqrt{\omega^2.\dfrac{1}{\omega^2}}+1=3

So, the minimum value is 3 \boxed{3} if and only if ω 2 = 1 \omega^2=1 .

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