A Problem with a Magical Solution from Secrets in Inequalities


A Problem with a Magical Solution from Secrets in Inequalities, problem


Let $a=3x,\,$ $b=2y,\,$ $c=z.\,$ Then $P=3x+2y+z=a+b+c\,$ and $a^2+3b^2+15c^2=abc.\,$

We'll make a double application of the weighted AM-GM inequality:

$\displaystyle \frac{\sum w_kx_k}{\sum w_k}\ge\sqrt[\sum w_k]{\prod x_k^{w_k}}.$

First, with $\displaystyle w_1=\frac{1}{2},\,$ $w_2=\frac{1}{3},\,$ $w_3=\frac{1}{6}\,$ $(w_1+w_2+w_3=1!),$


$\displaystyle a+b+c\ge (2a)^{\frac{1}{2}}(3b)^{\frac{1}{3}}(6c)^{\frac{1}{6}}.$

Then, with $\displaystyle w_1=\frac{1}{4},\,$ $\displaystyle w_2=\frac{3}{9}=\frac{1}{3},\,$ $\displaystyle w_3=\frac{15}{36}=\frac{5}{12}\,$ $(w_1+w_2+w_3=1!),$


$\displaystyle\begin{align}a^2+3b^2+15c^2&\ge (4a^2)^{\frac{1}{4}}(9b^2)^{\frac{3}{9}}(36c^2)^{\frac{15}{36}}\\ &=(4a^2)^{\frac{1}{4}}(9b^2)^{\frac{1}{3}}(36c^2)^{\frac{5}{12}}. \end{align}$

Multiplying (1) and (2),

$\displaystyle (a+b+c)(a^2+3b^2+15c^2)\ge 36abc,$

which implies $a+b+c\ge 36,\,$ the quantity that is attained for $x=y=z=6,\,$ making it the sought minimum.


This is problem #81 from Phan Kim Hungs' Secrets in Inequalities, (GIL Publishing House, 2007). I am grateful to Dan Sitaru who mailed me the problem and helped understand its solution.


Related material

A Sample of Optimization Problems III

  • Mathematicians Like to Optimize
  • Mathematics in Pizzeria
  • The Distance to Look Your Best
  • Building a Bridge
  • Linear Programming
  • Residence at an Optimal Distance
  • Distance Between Projections
  • Huygens' Problem
  • Optimization in a Crooked Trapezoid
  • Greatest Difference in Arithmetic Progression
  • Area Optimization in Trapezoid
  • Minimum under Two Constraints
  • Optimization with Many Variables
  • Minimum of a Cyclic Sum with Logarithms
  • Leo Giugiuc's Optimization with Constraint
  • Problem 4033 from Crux Mathematicorum
  • An Unusual Problem by Leo Giugiuc
  • A Cyclic Inequality With Constraint in Two Triples of Variables
  • Two Problems by Kunihiko Chikaya
  • An Inequality and Its Modifications
  • A 2-Variable Optimization From a China Competition
  • |Contact| |Up| |Front page| |Contents| |Algebra|

    Copyright © 1996-2018 Alexander Bogomolny