Maths Olympiad Prep

Library / /323 of 740

, 2022

Algebra Difficulty 4.9 AIME Prove it United States

Problem:
Suppose xx and yy are positive real numbers such that
x+1y=y+2x=3. x + \frac{1}{y} = y + \frac{2}{x} = 3.
Compute the maximum possible value of xyx y.

Solutions — 2

Solution 1

Solution:
Rewrite the equations as xy+1=3yx y + 1 = 3 y and xy+2=3xx y + 2 = 3 x. Let xy=Cx y = C, so x=C+23x = \frac{C+2}{3} and y=C+13y = \frac{C+1}{3}. Then
(C+23)(C+13)=CC26C+2=0. \left(\frac{C+2}{3}\right)\left(\frac{C+1}{3}\right) = C \Longrightarrow C^{2} - 6C + 2 = 0.
The larger of its two roots is 3+73 + \sqrt{7}.

Solution 2

Solution:
Multiply the two equations to get xy+3+2xy=9x y + 3 + \frac{2}{x y} = 9, so letting C=xyC = x y gives C26C+2=0C^{2} - 6C + 2 = 0, which has larger root C=3+7C = 3 + \sqrt{7}.

Want a route through all this instead of an archive? The track puts 2,000 problems in a working order, from AMC 10 level to the IMO shortlist.

Source: MathNet, licensed CC-BY-4.0. Statement reproduced verbatim; metadata (topic, difficulty) added by this project.