Inequality in disguise

Algebra Level 3

How many ( x , y ) (x,y) pairs are there in the set

A = { ( x , y ) log ( x 3 + y 3 3 + 1 9 ) = log x + log y } ? A= \left\{ (x,y) \ \bigg| \ \log \left(x^3+\frac{y^3}{3}+\frac{1}{9}\right)=\log x+\log y \right\} ?

2 More than 2 1 0

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1 solution

ChengYiin Ong
Nov 3, 2019

We have to solve the equation x 3 + y 3 3 + 1 9 = x y x^3+\frac{y^3}{3}+\frac{1}{9}=xy , it's kinda hard to solve by factoring, and sometimes it's better to invoke some inequalities in an equality! By AM-GM, x 3 + y 3 3 + 1 9 3 ( x 3 y 3 27 ) 1 3 = x y x^3+\frac{y^3}{3}+\frac{1}{9}\geq3(\ \frac{x^3y^3}{27})^\frac{1}{3}=xy , thus, there is only one element which is ( x , y ) = ( 1 9 1 3 , 1 3 1 3 ) (x,y)=(\frac{1}{9^\frac{1}{3}}, \frac{1}{3^\frac{1}{3}} ) by setting x 3 = y 3 3 = 1 9 x^3=\frac{y^3}{3}=\frac{1}{9} which is the point where the function x 3 + y 3 3 + 1 9 x^3+\frac{y^3}{3}+\frac{1}{9} achieves its minimum value x y xy .

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