Maths Olympiad Prep

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Algebra Difficulty 7.5 National olympiad, round 2 Prove it Baltic Way

Let a,b,c,da, b, c, d be positive numbers such that abcd=1abcd = 1. Prove the inequality
1a+2b+3c+10+1b+2c+3d+10+1c+2d+3a+10+1d+2a+3b+101. \frac{1}{\sqrt{a + 2b + 3c + 10}} + \frac{1}{\sqrt{b + 2c + 3d + 10}} + \frac{1}{\sqrt{c + 2d + 3a + 10}} + \frac{1}{\sqrt{d + 2a + 3b + 10}} \le 1.

Solution

Let x,y,z,tx, y, z, t be positive numbers such that a=x4,b=y4,c=z4,d=t4a = x^4, b = y^4, c = z^4, d = t^4.
By AM-GM inequality x4+y4+z4+14xyzx^4 + y^4 + z^4 + 1 \ge 4xyz, y4+z4+1+14yzy^4 + z^4 + 1 + 1 \ge 4yz and z4+1+1+14zz^4 + 1 + 1 + 1 \ge 4z.
Therefore we have the following estimation for the first fraction
1x4+2y4+3z4+1014xyz+4yz+4z+4=12xyz+yz+z+1. \frac{1}{\sqrt{x^4 + 2y^4 + 3z^4 + 10}} \le \frac{1}{\sqrt{4xyz + 4yz + 4z + 4}} = \frac{1}{2\sqrt{xyz + yz + z + 1}}.
Transform analogous estimations for the other fractions:
1b+2c+3d+1012yzt+zt+t+1=12tyz+z+1+xyz=xyz2xyz+yz+z+1;1c+2d+3a+1012ztx+tx+x+1=12txz+1+xyz+yz=yz2xyz+yz+z+1;1d+2a+3b+1012txy+xy+y+1=12txy1+xyz+yz+z=z2xyz+yz+z+1. \begin{aligned} \frac{1}{\sqrt{b + 2c + 3d + 10}} &\le \frac{1}{2\sqrt{yzt + zt + t + 1}} = \frac{1}{2\sqrt{t\sqrt{yz + z + 1 + xyz}}} = \frac{\sqrt{xyz}}{2\sqrt{xyz + yz + z + 1}}; \\ \frac{1}{\sqrt{c + 2d + 3a + 10}} &\le \frac{1}{2\sqrt{ztx + tx + x + 1}} = \frac{1}{2\sqrt{tx\sqrt{z + 1 + xyz + yz}}} = \frac{\sqrt{yz}}{2\sqrt{xyz + yz + z + 1}}; \\ \frac{1}{\sqrt{d + 2a + 3b + 10}} &\le \frac{1}{2\sqrt{txy + xy + y + 1}} = \frac{1}{2\sqrt{txy\sqrt{1 + xyz + yz + z}}} = \frac{\sqrt{z}}{2\sqrt{xyz + yz + z + 1}}. \end{aligned}
Thus, the sum does not exceed
1+xyz+yz+z2xyz+yz+z+1. \frac{1 + \sqrt{xyz} + \sqrt{yz} + \sqrt{z}}{2\sqrt{xyz + yz + z + 1}}.
It remains to apply inequality α+β+γ+δ2α+β+γ+δ\sqrt{\alpha} + \sqrt{\beta} + \sqrt{\gamma} + \sqrt{\delta} \le 2\sqrt{\alpha + \beta + \gamma + \delta}, which can be easily proven by taking squares or derived from inequality between arithmetical and quadratic means.

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Source: MathNet, licensed CC-BY-4.0. Statement and solution reproduced as published; topic and difficulty added by this site.