Equivalent Equations!

Algebra Level 2

Let x , y , z x,y,z be distinct real numbers satisfying x + 1 y = y + 1 z = z + 1 x . x + \dfrac1y = y + \dfrac1z = z + \dfrac1x . Find the value of x 2 y 2 z 2 x^2 y^2z^2 .


The answer is 1.

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1 solution

Toshit Jain
Mar 11, 2017

G i v e n , x + 1 y = y + 1 z = z + 1 x Given \space , \space x \space + \frac{1}{y} \space =\space y \space + \space \frac{1}{z} \space=\space z \space + \space \frac{1}{x}

x y = 1 z 1 y = y z y z ( 1 ) \rightarrow x \space - \space y \space=\space \frac{1}{z} \space - \space \frac{1}{y} \space=\space \frac{y-z}{yz} \space\space \space \boxed{(1)}

y z = 1 x 1 z = z x x z ( 2 ) \rightarrow y \space - \space z \space=\space \frac{1}{x} \space - \space \frac{1}{z} \space=\space \frac{z-x}{xz} \space\space\space \boxed{(2)}

z x = 1 y 1 x = x y x y ( 3 ) \rightarrow z \space - \space x \space=\space \frac{1}{y} \space - \space \frac{1}{x} \space=\space \frac{x-y}{xy} \space\space\space \boxed{(3)}

M u l t i p l y i n g ( 1 ) , ( 2 ) a n d ( 3 ) , w e g e t Multiplying \space (1) \space,\space (2) \space and \space (3) \space, \space we \space get

( x y ) ( y z ) ( z x ) = ( x y ) ( y z ) ( z x ) ( x y z ) 2 \Rightarrow (x-y)(y-z)(z-x) \space=\space \frac{(x-y)(y-z)(z-x)}{(xyz)^{2}}

x 2 y 2 z 2 = 1 \boxed{\therefore \space x^{2}y^{2}z^{2} \space=\space 1}

This is incomplete. You still need to show that there exists distinct real numbers x , y , z x,y,z that satisfy the constraints.

Pi Han Goh - 4 years, 3 months ago

The word you have used distinct is not suitible here because on solving I have found that the solution exists if and only if x = y = z x=y=z

Kushal Bose - 4 years, 3 months ago

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Agreed that for this problem to be well phrased, we need solution sets to the original expression.

As it turns out, there are non-disinct solutions. E.g. x = 1 , y = 1 / 2 , 2 x = -1, y = 1/2, 2 .

More generally, in addition to that solution and x = y = z = 1 x = y = z = 1 , the solution sets are of the form ( x , 1 x + 1 , x + 1 x ) , ( x , 1 1 x , x 1 x ) ( x, -\frac{1}{x+1}, \frac{x+1}{x}), ( x, \frac{1}{1-x}, \frac{x-1}{x} ) for x 0 , 1 , 1 x \neq 0, 1, -1 . Do you see how to arrive at this conclusion? (Assuming that I haven't made a mistake, which could be possible).

Calvin Lin Staff - 4 years, 3 months ago

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No, I am not getting.Can u provide any hint

Kushal Bose - 4 years, 3 months ago

@Kushal Bose Even I am also facing the same. But that's NMTC 2016 problem!

Toshit Jain - 4 years, 3 months ago

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