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Algebra Difficulty 5.6 AIME, harder Prove it Iran

Let aa, bb, cc and dd be positive real numbers with a+b+c+d=2a + b + c + d = 2. Prove that
(a+c)2ad+bc+(b+d)2ac+bd+44(a+b+1c+d+1+c+d+1a+b+1). \frac{(a+c)^2}{ad+bc} + \frac{(b+d)^2}{ac+bd} + 4 \ge 4 \left( \frac{a+b+1}{c+d+1} + \frac{c+d+1}{a+b+1} \right).

Solution

According to Cauchy-Schwarz inequality
((a+c)2ad+bc+(b+d)2ac+bd+4)((ad+bc)+(ac+bd)+1)(a+c+b+d+2)2=16    (a+c)2ad+bc+(b+d)2ac+bd+416ad+bc+ac+bd+1. \left( \frac{(a+c)^2}{ad+bc} + \frac{(b+d)^2}{ac+bd} + 4 \right) \left( (ad+bc) + (ac+bd) + 1 \right) \geq (a+c+b+d+2)^2 = 16 \implies \frac{(a+c)^2}{ad+bc} + \frac{(b+d)^2}{ac+bd} + 4 \geq \frac{16}{ad+bc+ac+bd+1}.
Set a+b=xa+b=x, c+d=yc+d=y (therefore x+y=2x+y=2, x2+y2=42xyx^2+y^2=4-2xy). It suffices to show that
16ad+bc+ac+bd+1=16xy+14(x+1y+1+y+1x+1). \frac{16}{ad + bc + ac + bd + 1} = \frac{16}{xy + 1} \geq 4 \left( \frac{x+1}{y+1} + \frac{y+1}{x+1} \right).
This is equivalent to
4(x+1)(y+1)(xy+1)((x+1)2+(y+1)2)4(xy+3)(xy+1)(102xy)4xy+128xy2(xy)2+10(xy)2+12xy(xy1)20. \begin{align*} 4(x+1)(y+1) &\geq (xy+1)((x+1)^2 + (y+1)^2) \\ \Leftrightarrow \quad 4(xy+3) &\geq (xy+1)(10-2xy) \\ \Leftrightarrow \quad 4xy+12 &\geq 8xy-2(xy)^2+10 \\ \Leftrightarrow \quad (xy)^2+1 &\geq 2xy \\ \Leftrightarrow \quad (xy-1)^2 &\geq 0. \end{align*}
Which is true, hence the claim of the problem. ■

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