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Difference between revisions of "2013 AMC 12A Problems/Problem 8"

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<math>xy = 2</math>
 
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== Solution 3 ==
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Let <math>A = x + \frac{2}{x} = y + \frac{2}{y}.</math> Consider the equation <cmath>u + \frac{2}{u} = A.</cmath> Reorganizing, we see that <math>u</math> satisfies <cmath>u^2 - Au + 2 = 0.</cmath> Notice that there can be at most two distinct values of <math>u</math> which satisfy this equation, and <math>x</math> and <math>y</math> are two distinct possible values for <math>u.</math> Therefore, <math>x</math> and <math>y</math> are roots of this quadratic, and by Vieta’s formulas we see that <math>xy</math> thereby must equal <math>\boxed{2}.</math>
  
 
== Video Solution ==
 
== Video Solution ==

Revision as of 18:57, 27 October 2021

Problem

Given that $x$ and $y$ are distinct nonzero real numbers such that $x+\tfrac{2}{x} = y + \tfrac{2}{y}$, what is $xy$?

$\textbf{(A)}\ \frac{1}{4}\qquad\textbf{(B)}\ \frac{1}{2}\qquad\textbf{(C)}\ 1\qquad\textbf{(D)}\ 2\qquad\textbf{(E)}\ 4\qquad$

Solution 1

$x+\tfrac{2}{x}= y+\tfrac{2}{y}$

Since $x\not=y$, we may assume that $x=\frac{2}{y}$ and/or, equivalently, $y=\frac{2}{x}$.

Cross multiply in either equation, giving us $xy=2$.

$\boxed{\textbf{(D) }{2}}$

Solution 2

$x+\tfrac{2}{x}= y+\tfrac{2}{y}$

$x-y+\frac{2}{x}-\frac{2}{y} = 0$

$(x-y)+2(\frac{y-x}{xy}) = 0$

$(x-y)(1-\frac{2}{xy})=0$

Since $x\not=y$

$1 = \frac{2}{xy}$

$xy = 2$

Solution 3

Let $A = x + \frac{2}{x} = y + \frac{2}{y}.$ Consider the equation \[u + \frac{2}{u} = A.\] Reorganizing, we see that $u$ satisfies \[u^2 - Au + 2 = 0.\] Notice that there can be at most two distinct values of $u$ which satisfy this equation, and $x$ and $y$ are two distinct possible values for $u.$ Therefore, $x$ and $y$ are roots of this quadratic, and by Vieta’s formulas we see that $xy$ thereby must equal $\boxed{2}.$

Video Solution

https://youtu.be/ba6w1OhXqOQ?t=1129

~ pi_is_3.14

Video Solution

https://youtu.be/CCjcMVtkVaQ ~sugar_rush

See also

2013 AMC 12A (Problems • Answer Key • Resources)
Preceded by
Problem 7 		Followed by
Problem 9
1 • 2 • 3 • 4 • 5 • 6 • 7 • 8 • 9 • 10 • 11 • 12 • 13 • 14 • 15 • 16 • 17 • 18 • 19 • 20 • 21 • 22 • 23 • 24 • 25
All AMC 12 Problems and Solutions

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