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find solution

Albert

Well-known member
Jan 25, 2013
1,225
find solutions of $x,y,z$

$\dfrac {1}{x}+\dfrac {1}{y+z}=\dfrac {1}{2}-----(1)$

$\dfrac {1}{y}+\dfrac {1}{z+x}=\dfrac {1}{3}-----(2)$

$\dfrac {1}{z}+\dfrac {1}{x+y}=\dfrac {1}{4}-----(3)$
 

soroban

Well-known member
Feb 2, 2012
409
Hello, Albert!

I have a very looong solution.


Find solutions of $x,y,z$

.[tex]\begin{array}{cccc}\frac {1}{x}+\frac {1}{y+z} &=& \frac {1}{2} & [1[ \\

\frac {1}{y}+\frac {1}{z+x}&=&\frac {1}{3} & [2] \\

\frac {1}{z}+\frac {1}{x+y}&=&\frac {1}{4} & (3) \end{array}[/tex]

Note that: .[tex]x,y,z\, \ne\,0.[/tex]

[tex]\begin{array}{cccccccccc}[1]\!:\;2y + 2x + 2x \:=\:x(y+z) &\Rightarrow& x + y + z \:=\: \frac{xy+xz}{2} & [4] \\
[2]\!:\; 3x + 3x + 3y \:=\:y(x+z) &\Rightarrow & x+y+z \:=\:\frac{xy+yz}{3} & [5] \\
[3]\!:\; 4x+3y+4z \:=\:z(x+y) & \Rightarrow & x+y+z \:=\:\frac{xz+yz}{4} & [6] \end{array}[/tex]

[tex]\text{From }[4],[5],[6]\!:\;\underbrace{\frac{xy+xz}{2}}_{[7]} \:=\:\underbrace{\frac{xy + yz}{3}}_{[8]} \:=\:\underbrace{\frac{xz+yz}{4}}_{[9]}[/tex]

[tex][7]=[8]\!:\;3xy + 3xz \:=\:2xy + 2yz \;\;\;\Rightarrow\;\;\; x \:=\:\frac{2yz}{y+3x}\;\;[10][/tex]

[tex][8] = [9]\!:\;4xy + 4yz \:=\:3xz + 3yz \;\;\;\Rightarrow\;\;\;x \:=\:\frac{yz}{3x-4y}\;\;[11][/tex]

[tex][10]=[11]\!:\;\frac{2yz}{y+3x} \:=\:\frac{yz}{3z-4y} \quad\Rightarrow\quad z \:=\:3y\;\;[12][/tex]

Substitute into [10]: .[tex]x \:=\:\frac{2y(3y)}{y+3(3y)} \quad\Rightarrow\quad x \:=\:\tfrac{3}{5}y\;\;[13][/tex]

Substitute [12] and [13] into [4]:
.. [tex]2y + 2(3y) + 2\left(\tfrac{3}{5}y\right) \;=\; \left(\tfrac{3}{5}y\right)y + \left(\tfrac{3}{5}y\right)(3y) [/tex]
. . [tex]\tfrac{46}{5}y \:=\:\tfrac{12}{5}y^2 \quad\Rightarrow\quad 6y^2\:=\:23y \quad\Rightarrow\quad \boxed{y \:=\:\tfrac{23}{6}}[/tex]

Substitute onto [13}: .[tex]x \:=\:\tfrac{3}{5}\left(\tfrac{23}{6}\right) \quad\Rightarrow\quad \boxed{x \:=\:\frac{23}{10}}[/tex]

Substitute into [12]: .[tex]z \:=\:3\left(\tfrac{23}{6}\right) \quad\Rightarrow\quad \boxed{z \:=\:\frac{23}{2}}[/tex]
 

Opalg

MHB Oldtimer
Staff member
Feb 7, 2012
2,703