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Algebra Difficulty 6.1 National olympiad Prove it Ukraine

For positive numbers aa, bb, cc, such that abc=1abc = 1, prove the inequality:
a2+b2c2+a+b+b2+c2a2+b+c+c2+a2b2+c+a2. \frac{a^2 + b^2}{c^2 + a + b} + \frac{b^2 + c^2}{a^2 + b + c} + \frac{c^2 + a^2}{b^2 + c + a} \le 2.

Solution

abc=113(a+b+c)abc = 1 \Rightarrow \frac{1}{3}(a+b+c). Then:
c2+a+bc2+(a+b)13(a+b+c)=13(3c2+a2+b2+2ab+ac+bc)16(8c2+5a2+5b2). \begin{aligned} c^2 + a + b &\le c^2 + (a+b)\frac{1}{3}(a+b+c) \\ &= \frac{1}{3}(3c^2 + a^2 + b^2 + 2ab + ac + bc) \\ &\le \frac{1}{6}(8c^2 + 5a^2 + 5b^2). \end{aligned}
So
a2+b2c2+a+b+a2+b216(8c2+5a2+5b2)++6(a2+b2)8c2+5a2+5b2+2, \frac{a^2 + b^2}{c^2 + a + b} + \dots \ge \frac{a^2 + b^2}{\frac{1}{6}(8c^2 + 5a^2 + 5b^2)} + \dots + \frac{6(a^2 + b^2)}{8c^2 + 5a^2 + 5b^2} + \dots \ge 2,
the last inequality must be proved. As three points meant a cyclic permutation of variables. For simplification denote x=a2x = a^2, y=b2y = b^2, z=c2z = c^2. Then we need to prove such inequality:
A=x+y8z+5x+5y+13. A = \frac{x+y}{8z+5x+5y} + \dots \ge \frac{1}{3}.
A=(x+y)2(x+y)(8z+5x+5y)+((x+y)2+)2(x+y)(8z+5x+5y)=4p+8q10p+26q13, A = \frac{(x+y)^2}{(x+y)(8z+5x+5y)} + \dots \ge \frac{((x+y)^2 + \dots)^2}{(x+y)(8z+5x+5y)} = \frac{4p+8q}{10p+26q} \ge \frac{1}{3},
this inequality supervenes from Cauchy-Schwarz inequality. There are used next denote: p=x2+y2+z2p = x^2 + y^2 + z^2, q=xy+yz+zxq = xy + yz + zx. The last inequality equivalent pqp \ge q, which are well known.

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