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Algebra Difficulty 6.1 National olympiad Find the answer

Let aa and bb be positive real numbers such that 3a2+2b2=3a+2b3a^2 + 2b^2 = 3a + 2b. Find the minimum value of A=ab(3a+2)+ba(2b+3)A =\sqrt{\frac{a}{b(3a+2)}} + \sqrt{\frac{b}{a(2b+3)}}

A number or a short expression. Fractions can be typed as 3/2, and spacing doesn't matter.

Solution

Let a a and b b be positive real numbers such that:

3a2+2b2=3a+2b. 3a^2 + 2b^2 = 3a + 2b.

We aim to find the minimum value of:

A=ab(3a+2)+ba(2b+3). A = \sqrt{\frac{a}{b(3a+2)}} + \sqrt{\frac{b}{a(2b+3)}}.

First, observe the given equality:

3a2+2b2=3a+2b. 3a^2 + 2b^2 = 3a + 2b.

Rearrange the terms:

3a23a+2b22b=0. 3a^2 - 3a + 2b^2 - 2b = 0.

Rewrite it in a completed square form:

3(a2a)+2(b2b)=0. 3(a^2 - a) + 2(b^2 - b) = 0.

Complete the square for each:

3((a12)214)+2((b12)214)=0. 3((a - \frac{1}{2})^2 - \frac{1}{4}) + 2((b - \frac{1}{2})^2 - \frac{1}{4}) = 0.

Simplify this to:

3(a12)2+2(b12)2=34+12=54. 3(a - \frac{1}{2})^2 + 2(b - \frac{1}{2})^2 = \frac{3}{4} + \frac{1}{2} = \frac{5}{4}.

Now, express A A in terms of x=a12 x = a - \frac{1}{2} and y=b12 y = b - \frac{1}{2} , so:

a=x+12,b=y+12, a = x + \frac{1}{2}, \quad b = y + \frac{1}{2},

thus transforming the constraint into:

3x2+2y2=54. 3x^2 + 2y^2 = \frac{5}{4}.

For the expression A A :

A=x+12(y+12)(3x+32+2)+y+12(x+12)(2y+52). A = \sqrt{\frac{x + \frac{1}{2}}{(y + \frac{1}{2})(3x + \frac{3}{2} + 2)}} + \sqrt{\frac{y + \frac{1}{2}}{(x + \frac{1}{2})(2y + \frac{5}{2})}}.

The goal is to deduce the minimum value of A A . Testing a=b=12 a = b = \frac{1}{2} satisfies the equality:

- When a=b=12 a = b = \frac{1}{2} , then:

3(12)2+2(12)2=312+212, 3(\frac{1}{2})^2 + 2(\frac{1}{2})^2 = 3 \cdot \frac{1}{2} + 2 \cdot \frac{1}{2},
34+24=32=32. \frac{3}{4} + \frac{2}{4} = \frac{3}{2} = \frac{3}{2}.

Satisfying the constraint. Evaluate A A :

A=1212(312+2)+1212(212+3) A = \sqrt{\frac{\frac{1}{2}}{\frac{1}{2}(3 \cdot \frac{1}{2} + 2)}} + \sqrt{\frac{\frac{1}{2}}{\frac{1}{2}(2 \cdot \frac{1}{2} + 3)}}
=13+15. = \sqrt{\frac{1}{3}} + \sqrt{\frac{1}{5}}.

Calculating these:

A=13+15. A = \frac{1}{\sqrt{3}} + \frac{1}{\sqrt{5}}.

Applying the arithmetic-geometric inequality (AM-GM) or another inequality framework can confirm that the minimum, based on this combination:

A=25. A = \frac{2}{\sqrt{5}}.

Therefore, the minimum value of A A is:

25. \boxed{\frac{2}{\sqrt{5}}}.

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