Maths Olympiad Prep

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Algebra Difficulty 5.6 AIME, harder Prove it North Macedonia

Let aa, bb, cc be positive real numbers such that a+b+c=3a + b + c = 3. Find the minimum value of the expression
A=2a3a+2b3b+2c3c A = \frac{2 - a^3}{a} + \frac{2 - b^3}{b} + \frac{2 - c^3}{c}

Solution

A=2a3a+2b3b+2c3c=2(1a+1b+1c)a2b2c2=2ab+bc+caabc(a2+b2+c2)=2ab+bc+caabc((a+b+c)22(ab+bc+ca))=2ab+bc+caabc(92(ab+bc+ca))=2ab+bc+caabc+2(ab+bc+ca)9=2(ab+bc+ca)(1abc+1)9\begin{align*} A &= \frac{2 - a^3}{a} + \frac{2 - b^3}{b} + \frac{2 - c^3}{c} = 2\left(\frac{1}{a} + \frac{1}{b} + \frac{1}{c}\right) - a^2 - b^2 - c^2 \\ &= 2\frac{ab + bc + ca}{abc} - (a^2 + b^2 + c^2) \\ &= 2\frac{ab + bc + ca}{abc} - \left((a + b + c)^2 - 2(ab + bc + ca)\right) \\ &= 2\frac{ab + bc + ca}{abc} - (9 - 2(ab + bc + ca)) \\ &= 2\frac{ab + bc + ca}{abc} + 2(ab + bc + ca) - 9 \\ &= 2(ab + bc + ca)\left(\frac{1}{abc} + 1\right) - 9 \end{align*}

Recall now the well-known inequality (x+y+z)23(xy+yz+zx)(x + y + z)^2 \ge 3(xy + yz + zx) and set x=abx = ab, y=bcy = bc, z=caz = ca, to obtain (ab+bc+ca)23abc(a+b+c)=9abc(ab + bc + ca)^2 \ge 3abc(a + b + c) = 9abc where we have used a+b+c=3a + b + c = 3. By taking the square roots on both sides of the last one we obtain:
ab+bc+ca3abc.(1) ab + bc + ca \geq 3\sqrt{abc}. \tag{1}
Also by using AM-GM inequality we get that
1abc+121abc.(2) \frac{1}{abc} + 1 \ge 2\sqrt{\frac{1}{abc}}. \tag{2}
Multiplication of (1) and (2) gives
(ab+bc+ca)(1abc+1)3abc21abc=6. (ab + bc + ca)\left(\frac{1}{abc} + 1\right) \ge 3\sqrt{abc} \cdot 2\sqrt{\frac{1}{abc}} = 6.
So A269=3A \ge 2 \cdot 6 - 9 = 3 and the equality holds if and only if a=b=c=1a = b = c = 1, so the minimum value is 33.

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Source: MathNet, licensed CC-BY-4.0. Statement reproduced verbatim; metadata (topic, difficulty) added by this project.