Maths Olympiad Prep

Library / /17 of 28

, 2024

Algebra Difficulty 5.3 AIME, harder Prove it United States

Problem:
Let x<yx < y be positive real numbers such that
x+y=4andx+2+y+2=5 \sqrt{x} + \sqrt{y} = 4 \quad \text{and} \quad \sqrt{x+2} + \sqrt{y+2} = 5
Compute xx.

Solution

Solution:
Adding and subtracting both equations gives
x+2+x+y+2+y=9x+2x+y+2y=1 \begin{aligned} & \sqrt{x+2} + \sqrt{x} + \sqrt{y+2} + \sqrt{y} = 9 \\ & \sqrt{x+2} - \sqrt{x} + \sqrt{y+2} - \sqrt{y} = 1 \end{aligned}
Substitute a=x+x+2a = \sqrt{x} + \sqrt{x+2} and b=y+y+2b = \sqrt{y} + \sqrt{y+2}. Then since (x+2+x)(x+2x)=2(\sqrt{x+2} + \sqrt{x})(\sqrt{x+2} - \sqrt{x}) = 2, we have
a+b=92a+2b=1 \begin{gathered} a + b = 9 \\ \frac{2}{a} + \frac{2}{b} = 1 \end{gathered}
Dividing the first equation by the second one gives
ab=18,a=3,b=6 ab = 18, \quad a = 3, \quad b = 6
Lastly, x=x+2+x(x+2x)2=3232=76\sqrt{x} = \frac{\sqrt{x+2} + \sqrt{x} - (\sqrt{x+2} - \sqrt{x})}{2} = \frac{3 - \frac{2}{3}}{2} = \frac{7}{6}, so x=4936x = \frac{49}{36}.

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.