2022 SSMO Speed Round Problems/Problem 9

Revision as of 13:00, 3 July 2023 by Pinkpig (talk | contribs) (Created page with "==Problem== Find the sum of the maximum and minimum values of <math>8x^2+7xy+5y^2</math> under the constraint that <math>3x^2+5xy+3y^2 = 88.</math> ==Solution== We want to m...")
(diff) ← Older revision | Latest revision (diff) | Newer revision → (diff)

Problem

Find the sum of the maximum and minimum values of $8x^2+7xy+5y^2$ under the constraint that $3x^2+5xy+3y^2 = 88.$

Solution

We want to maximize $k$ such \[8x^2+7xy+5y^2 = \frac{k}{88}(3x^2+5xy+3y^2)\] or, if $a = \frac{x}{y}$ \[8a^2+7a+5 = \frac{k}{88}(3a^2+5a+3)\] which has discriminant in $a$ of \[121(-k^2 + 688k - 78144)\] so the sum of the extremes of $k$ are $\boxed{688}$.