An algebra problem by Anas Kudsi

Algebra Level 3

Let a a , b b , and c c be positive real numbers satisfying a b + b c + c a = 1 ab + bc + ca = 1 .

Find the minimum value of cyclic a 10 ( b 3 + c 3 ) ( b + c ) \displaystyle \sum_{\text{cyclic}} \dfrac{a^{10}}{(b^3 + c^3)(b+c)} .

1 36 \frac1{36} 1 18 \frac1{18} 2 9 \frac29 1 4 \frac14

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1 solution

Anas Kudsi
Feb 15, 2018

WLOG assume that a + b a + c b + c a+b\ge a+c\ge b+c then using Chebyshev have: c y c a 10 ( b + c ) ( b 3 + c 3 ) 1 3 \sum_{cyc}\frac{a^{10}}{(b+c) (b^3+c^3)}\ge \frac{1}{3} ( c y c a 6 b 3 + c 3 ) ( c y c a 4 b + c ) 1 3 a 3 + b 3 + c 3 2 ( a 2 + b 2 + c 2 ) 2 2 ( a + b + c ) (\sum_{cyc}\frac{a^6}{b^3+c^3})(\sum_{cyc}\frac{a^4}{b+c})\ge \frac{1}{3}\frac{a^3+b^3+c^3}{2}\frac{(a^2+b^2+c^2)^2}{2(a+b+c)} also have ( a + b + c ) 2 3 ( a b + b c + a c ) = 3 (a+b+c)^2\ge 3(ab+bc+ac)=3 and by Hölder 9 ( a 3 + b 3 + c 3 ) ( a + b + c ) 3 9(a^3+b^3+c^3)\ge(a+b+c)^3 hence 1 3 a 3 + b 3 + c 3 2 ( a 2 + b 2 + c 2 ) 2 2 ( a + b + c ) 1 3 ( ( a + b + c ) 3 18 ) 2 9 1 8 2 = 1 36 \frac{1}{3}\frac{a^3+b^3+c^3}{2}\frac{(a^2+b^2+c^2)^2}{2(a+b+c)}\ge \frac{1}{3}(\frac{(a+b+c)^3}{18})^2\ge\frac{9}{18^2}=\frac{1}{36} So the minimum is 1 36 \frac{1}{36} Done.

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