Solution: By A M − G M AM-GM A M − GM we have 2 x 2 + 1 x = x 2 + x 2 + 1 x ≥ 3 x 4 x 3 = 3 x 2x^{2}+\frac{1}{x}=x^{2}+x^{2}+\frac{1}{x} \geq 3 \sqrt[3]{\frac{x^{4}}{x}}=3x 2 x 2 + x 1 = x 2 + x 2 + x 1 ≥ 3 3 x x 4 = 3 x for all x > 0 x>0 x > 0 , so we have:
∑ cyc 2 a 2 + 1 a b + 1 a + 1 ≥ ∑ c y c 3 a 1 + b + b c = 3 ( ∑ c y c a 2 1 + a + a b ) ≥ 3 ( a + b + c ) 2 3 + a + b + c + a b + b c + c a \sum_{\text{cyc}} \frac{2 a^{2}+\frac{1}{a}}{b+\frac{1}{a}+1} \geq \sum_{cyc} \frac{3a}{1+b+bc}=3\left(\sum_{cyc} \frac{a^{2}}{1+a+ab}\right) \geq \frac{3(a+b+c)^{2}}{3+a+b+c+ab+bc+ca} ∑ cyc b + a 1 + 1 2 a 2 + a 1 ≥ ∑ cy c 1 + b + b c 3 a = 3 ( ∑ cy c 1 + a + ab a 2 ) ≥ 3 + a + b + c + ab + b c + c a 3 ( a + b + c ) 2 .
By A M − G M AM-GM A M − GM we have a b + b c + c a ≥ 3 ab+bc+ca \geq 3 ab + b c + c a ≥ 3 and a + b + c ≥ 3 a+b+c \geq 3 a + b + c ≥ 3 . But 3 ( a 2 + b 2 + c 2 ) ≥ ( a + b + c ) 2 ≥ 3 ( a + b + c ) 3\left(a^{2}+b^{2}+c^{2}\right) \geq (a+b+c)^{2} \geq 3(a+b+c) 3 ( a 2 + b 2 + c 2 ) ≥ ( a + b + c ) 2 ≥ 3 ( a + b + c ) . So ( a + b + c ) 2 = a 2 + b 2 + c 2 + 2 a b + 2 b c + 2 c a ≥ 3 + a + b + c + a b + b c + c a (a+b+c)^{2}=a^{2}+b^{2}+c^{2}+2ab+2bc+2ca \geq 3+a+b+c+ab+bc+ca ( a + b + c ) 2 = a 2 + b 2 + c 2 + 2 ab + 2 b c + 2 c a ≥ 3 + a + b + c + ab + b c + c a . Hence ∑ c y c 2 a 2 + 1 a b + 1 a + 1 ≥ 3 ( a + b + c ) 2 3 + a + b + c + a b + b c + c a ≥ 3 ( a + b + c ) 2 ( a + b + c ) 2 = 3 \sum_{cyc} \frac{2 a^{2}+\frac{1}{a}}{b+\frac{1}{a}+1} \geq \frac{3(a+b+c)^{2}}{3+a+b+c+ab+bc+ca} \geq \frac{3(a+b+c)^{2}}{(a+b+c)^{2}}=3 ∑ cy c b + a 1 + 1 2 a 2 + a 1 ≥ 3 + a + b + c + ab + b c + c a 3 ( a + b + c ) 2 ≥ ( a + b + c ) 2 3 ( a + b + c ) 2 = 3 .
Denote a = y x a=\frac{y}{x} a = x y , b = z y b=\frac{z}{y} b = y z and c = x z c=\frac{x}{z} c = z x . We have 2 a 2 + 1 a b + 1 a + 1 = 2 y 2 x 2 + x y z y + x y + 1 = 2 y 3 + x 3 x 2 ( x + y + z ) \frac{2 a^{2}+\frac{1}{a}}{b+\frac{1}{a}+1}=\frac{\frac{2 y^{2}}{x^{2}}+\frac{x}{y}}{\frac{z}{y}+\frac{x}{y}+1}=\frac{2 y^{3}+x^{3}}{x^{2}(x+y+z)} b + a 1 + 1 2 a 2 + a 1 = y z + y x + 1 x 2 2 y 2 + y x = x 2 ( x + y + z ) 2 y 3 + x 3 .
Hence ∑ c y c 2 a 2 + 1 a b + 1 a + 1 = 1 x + y + z ⋅ ∑ c y c 2 y 3 + x 3 x 2 = 1 x + y + z ⋅ ( x + y + z + 2 ∑ c y c y 3 x 2 ) \sum_{cyc} \frac{2 a^{2}+\frac{1}{a}}{b+\frac{1}{a}+1}=\frac{1}{x+y+z} \cdot \sum_{cyc} \frac{2 y^{3}+x^{3}}{x^{2}}=\frac{1}{x+y+z} \cdot\left(x+y+z+2 \sum_{cyc} \frac{y^{3}}{x^{2}}\right) ∑ cy c b + a 1 + 1 2 a 2 + a 1 = x + y + z 1 ⋅ ∑ cy c x 2 2 y 3 + x 3 = x + y + z 1 ⋅ ( x + y + z + 2 ∑ cy c x 2 y 3 ) .
By Rearrangement Inequality we get ∑ cyc y 3 x 2 ≥ x + y + z \sum_{\text{cyc}} \frac{y^{3}}{x^{2}} \geq x+y+z ∑ cyc x 2 y 3 ≥ x + y + z .
So ∑ cyc 2 a 2 + 1 a b + 1 a + 1 ≥ 1 x + y + z ⋅ ( 3 x + 3 y + 3 z ) = 3 \sum_{\text{cyc}} \frac{2 a^{2}+\frac{1}{a}}{b+\frac{1}{a}+1} \geq \frac{1}{x+y+z} \cdot(3x+3y+3z)=3 ∑ cyc b + a 1 + 1 2 a 2 + a 1 ≥ x + y + z 1 ⋅ ( 3 x + 3 y + 3 z ) = 3 .