An algebra problem by P C

Algebra Level 5

c y c a , b , c a a 2 + 2 \large\displaystyle\sum_{cyc}^{a,b,c}\frac{a}{a^2+2} Let a , b , c > 0 a,b,c>0 satisfy a b c = 1 abc=1 , find the maximum value of the expression above.

Submit your answer to 2 decimal places


The answer is 1.00.

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1 solution

P C
Jun 11, 2016

Call the expression E, we rewrite it like this: E = c y c a , b , c 1 a + 2 a E=\displaystyle\sum_{cyc}^{a,b,c}\frac{1}{a+\frac{2}{a}} . By AM-GM E c y c a , b , c 1 2 + 1 a E\leq\displaystyle\sum_{cyc}^{a,b,c}\frac{1}{2+\frac{1}{a}} Set x = 1 a , y = 1 b , z = 1 c x=\frac{1}{a}, y=\frac{1}{b}, z=\frac{1}{c} then x y z = 1 xyz=1 and the R H S RHS becomes c y c x , y , z 1 2 + x \displaystyle\sum_{cyc}^{x,y,z}\frac{1}{2+x} . Now we consider 3 2 R H S = c y c x , y , z x 2 + x 3-2RHS=\sum_{cyc}^{x,y,z}\frac{x}{2+x} By Titu's Lemma 3 2 R H S ( x + y + z ) 2 x + y + z + 6 3-2RHS\geq\frac{(\sqrt{x}+\sqrt{y}+\sqrt{z})^2}{x+y+z+6} We see that x y + y z + z x A M G M 3 x y z 3 = 3 \sqrt{xy}+\sqrt{yz}+\sqrt{zx}\stackrel{AM-GM}\geq 3\sqrt[3]{xyz}=3 x + y + z + 2 ( x y + y z + z x ) = ( x + y + z ) 2 x + y + z + 6 \Rightarrow x+y+z+2(\sqrt{xy}+\sqrt{yz}+\sqrt{zx})=(\sqrt{x}+\sqrt{y}+\sqrt{z})^2\geq x+y+z+6 3 2 R H S 1 R H S 1 \therefore 3-2RHS\geq 1\Rightarrow RHS\leq 1 E 1 \therefore E\leq 1 The equality holds when a = b = c = 1 a=b=c=1

Note : There's a much shorter solution using U.C.T

How do you get the idea that 3-2rhs must be seen?

Tarun B - 3 years, 8 months ago

@P C What is UCT???

Aaghaz Mahajan - 2 years, 11 months ago

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