Vol. 6 · When does equality hold?

The Mean Inequalities and the Cauchy Inequality

A companion to 《平均值不等式与柯西不等式》 by Li Shenghong · 15 stops ·

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  1. §1.1The Mean InequalitiesFour means in a row: harmonic ≤ geometric ≤ arithmetic ≤ quadratic.Concept
  2. §1.2Proofs of the Mean InequalitiesKeep the sum, pull one number to the mean: the product never drops.ConceptM3M17
  3. §2.1Proving Inequalities with AM–GMWrite each small-side term as a weighted mean of big-side terms; add cyclically.Skill
  4. §2.2Extrema with AM–GMPositive, fixed, equal: split terms to fix the product, then check equality.SkillM6M17
  5. §2.3AM–GM in Geometric InequalitiesTangent lengths x, y, z sum to p; AM–GM on them, equality when equilateral.SkillM11
  6. §2.4Variants of AM–GMReciprocals, squares, weights: a whole family, ordered by the power mean.Skill
  7. §2.5AM–GM with ParametersPut in an unknown parameter, then choose it so all equality cases meet.Skill
  8. §3.1The Cauchy Inequality and Its Proofs(Σaᵢbᵢ)² ≤ Σaᵢ²·Σbᵢ²; the gap is a squared area, zero only when flattened.Concept
  9. §3.2Variants and Generalizations of CauchyMerging fractions into one can only lower the sum; equal ratios lose nothing.Concept
  10. §4.1Proving Inequalities with CauchyBuild two lists whose pairwise products lose every variable, leaving a constant.SkillM18
  11. §4.2Cauchy for Systems and ExtremaToo few equations? When Cauchy is forced to equality, the solution is pinned.Skill
  12. §4.3Cauchy and Fractional InequalitiesMake each numerator a square, then merge once. What you multiply by decides it.Skill
  13. §4.4Cauchy in Combinatorial EstimatesCount in two ways to bound Σdᵢ², then let Cauchy turn it into a bound on Σdᵢ.MethodM16M20
  14. §4.5Cauchy with ParametersInsert a weight λ. Every λ gives a bound; one λ kills x and is reached.Skill
  15. §4.6AM–GM and Cauchy TogetherChained bounds give the extremum only if every equality holds at the same point.MethodM9