Maths Olympiad Prep

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Algebra Difficulty 5.9 AIME, harder Prove it JBMO

Problem:

Let aa, bb, cc be positive real numbers such that a+b+c=1a + b + c = 1. Prove that
7+2b1+a+7+2c1+b+7+2a1+c694. \frac{7 + 2b}{1 + a} + \frac{7 + 2c}{1 + b} + \frac{7 + 2a}{1 + c} \geq \frac{69}{4}.
When does equality hold?

Solution

Solution:

The inequality can be written as:
5+2(1+b)1+a+5+2(1+c)1+b+5+2(1+a)1+c694. \frac{5 + 2(1 + b)}{1 + a} + \frac{5 + 2(1 + c)}{1 + b} + \frac{5 + 2(1 + a)}{1 + c} \geq \frac{69}{4}.
We substitute 1+a=x1 + a = x, 1+b=y1 + b = y, 1+c=z1 + c = z.
So, we have to prove the inequality
5+2yx+5+2zy+5+2xz6945(1x+1y+1z)+2(yx+zy+xz)694 \frac{5 + 2y}{x} + \frac{5 + 2z}{y} + \frac{5 + 2x}{z} \geq \frac{69}{4} \Leftrightarrow 5\left(\frac{1}{x} + \frac{1}{y} + \frac{1}{z}\right) + 2\left(\frac{y}{x} + \frac{z}{y} + \frac{x}{z}\right) \geq \frac{69}{4}
where x,y,z>1x, y, z > 1 real numbers and x+y+z=4x + y + z = 4.
We have
- x+y+z331x+1y+1z1x+1y+1z9x+y+z1x+1y+1z94\frac{x + y + z}{3} \geq \frac{3}{\frac{1}{x} + \frac{1}{y} + \frac{1}{z}} \Leftrightarrow \frac{1}{x} + \frac{1}{y} + \frac{1}{z} \geq \frac{9}{x + y + z} \Leftrightarrow \frac{1}{x} + \frac{1}{y} + \frac{1}{z} \geq \frac{9}{4}
- yx+zy+xz3yxzyxz3=3\frac{y}{x} + \frac{z}{y} + \frac{x}{z} \geq 3 \cdot \sqrt[3]{\frac{y}{x} \cdot \frac{z}{y} \cdot \frac{x}{z}} = 3

Thus,
5+2yx+5+2zy+5+2xz=5(1x+1y+1z)+2(yx+zy+xz)594+23=694. \frac{5 + 2y}{x} + \frac{5 + 2z}{y} + \frac{5 + 2x}{z} = 5\left(\frac{1}{x} + \frac{1}{y} + \frac{1}{z}\right) + 2\left(\frac{y}{x} + \frac{z}{y} + \frac{x}{z}\right) \geq 5 \cdot \frac{9}{4} + 2 \cdot 3 = \frac{69}{4}.
The equality holds when (x=y=z,yx=zy=xz,x+y+z=4)(x = y = z, \frac{y}{x} = \frac{z}{y} = \frac{x}{z}, x + y + z = 4), thus x=y=z=43x = y = z = \frac{4}{3}, i.e. a=b=c=13a = b = c = \frac{1}{3}.

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