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Algebra Difficulty 5.3 AIME, harder Find the answer

Let a,b,ca,b,c 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=\dfrac{2-a^3}a+\dfrac{2-b^3}b+\dfrac{2-c^3}c.

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

Solution

To 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},
given that a,b,c a, b, c are positive real numbers and a+b+c=3 a + b + c = 3 , we proceed as follows:

First, we rewrite the expression:
A=2aa2+2bb2+2cc2. A = \frac{2}{a} - a^2 + \frac{2}{b} - b^2 + \frac{2}{c} - c^2.

Consider the function of a single variable:
f(x)=2xx2. f(x) = \frac{2}{x} - x^2.
Our goal is to minimize the sum f(a)+f(b)+f(c) f(a) + f(b) + f(c) .

We calculate the derivative of f(x) f(x) :
f(x)=2x22x. f'(x) = -\frac{2}{x^2} - 2x.
Setting f(x)=0 f'(x) = 0 to find critical points:
2x22x=02x2=2xx3=1. -\frac{2}{x^2} - 2x = 0 \quad \Rightarrow \quad \frac{2}{x^2} = -2x \quad \Rightarrow \quad x^3 = 1.
Thus, x=1 x = 1 is a critical point where f(x) f(x) is extremized.

Checking the nature of this critical point, we compute the second derivative:
f(x)=4x32. f''(x) = \frac{4}{x^3} - 2.
Evaluating f(1) f''(1) :
f(1)=42=2>0. f''(1) = 4 - 2 = 2 > 0.
This indicates that x=1 x = 1 is a point of local minimum for f(x) f(x) .

Considering the condition a+b+c=3 a + b + c = 3 , a natural symmetry suggests a=b=c=1 a = b = c = 1 . Substituting these values back into the expression for A A :
A=(2112)+(2112)+(2112)=(21)+(21)+(21)=3. A = \left( \frac{2}{1} - 1^2 \right) + \left( \frac{2}{1} - 1^2 \right) + \left( \frac{2}{1} - 1^2 \right) = (2 - 1) + (2 - 1) + (2 - 1) = 3.

Therefore, the minimum value of A A is:
3. \boxed{3}.
This minimum is achieved when a=b=c=1 a = b = c = 1 .

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Source: Omni-MATH, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.