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

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<cmath> xy(x-y) = 2(x-y) </cmath>
 
<cmath> xy(x-y) = 2(x-y) </cmath>
  
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Divide by <math>(x-y)</math> {x and y} to get
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Divide by <math>(x-y)</math> {x and y are distinct} to get
  
 
<math>\boxed{2}.</math>
 
<math>\boxed{2}.</math>

Revision as of 21:12, 7 March 2024

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 $\boxed{2}.$

~ Professor-Mom

Solution 4

\[x + \frac{2}{x} = y + \frac{2}{y}.\]

Multiply both sides by xy to get

\[x^2y + 2y = y^2x +2x\]

Rearrange to get

\[x^2y - y^2x = 2x - 2y\]

Factor out $xy$ on the left side and $2$ on the right side to get

\[xy(x-y) = 2(x-y)\]

Divide by $(x-y)$ {x and y are distinct} to get

$\boxed{2}.$

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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