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avatar+12531 
0
Mar 26, 2020
 #1
avatar+226 
+1
Mar 26, 2020
 #10
avatar+26388 
+1

The system of equations \(\dfrac{xy}{x + y} = 1, \quad \dfrac{xz}{x + z} = 2, \quad \dfrac{yz}{y + z} = 3\)
has one ordered triple solution \((x,y,z)\).

What is the value of in this solution?

 

\(\begin{array}{|rcll|} \hline \dfrac{xy}{x + y} &=& 1 \\\\ \dfrac{x + y}{xy} &=& 1 \\\\ \dfrac{x}{xy}+\dfrac{y}{xy} &=& 1 \\\\ \mathbf{\dfrac{1}{y}+\dfrac{1}{x}} &=& \mathbf{1} \qquad (1) \\ \hline \end{array} \begin{array}{|rcll|} \hline \dfrac{xz}{x + z} &=& 2 \\\\ \dfrac{x + z}{xz} &=& \dfrac{1}{2} \\\\ \dfrac{x}{xz}+\dfrac{z}{xz} &=& \dfrac{1}{2} \\\\ \mathbf{\dfrac{1}{z}+\dfrac{1}{x}} &=& \mathbf{\dfrac{1}{2}} \qquad (2) \\ \hline \end{array} \begin{array}{|rcll|} \hline \dfrac{yz}{y + z} &=& 3 \\\\ \dfrac{y + z}{yz} &=& \dfrac{1}{3} \\\\ \dfrac{y}{yz}+\dfrac{z}{yz} &=& \dfrac{1}{3} \\\\ \mathbf{\dfrac{1}{z}+\dfrac{1}{y}} &=& \mathbf{\dfrac{1}{3}} \qquad (3) \\ \hline \end{array}\)

 

\(\begin{array}{|lrcll|} \hline -(1)+(2)+(3) : & -\dfrac{1}{y}-\dfrac{1}{x}+ \dfrac{1}{z}+\dfrac{1}{x}+ \dfrac{1}{z}+\dfrac{1}{y} &=& -1+\dfrac{1}{2}+\dfrac{1}{3} \\\\ & \dfrac{2}{z} &=& \dfrac{-1}{6} \\\\ & \dfrac{z}{2} &=& -6 \\\\ & \mathbf{z} &=& \mathbf{-12} \\ \hline \end{array} \\ \begin{array}{|lrcll|} \hline (1)-(2)+(3) : & \dfrac{1}{y}+\dfrac{1}{x}- \dfrac{1}{z}-\dfrac{1}{x}+ \dfrac{1}{z}+\dfrac{1}{y} &=& 1-\dfrac{1}{2}+\dfrac{1}{3} \\\\ & \dfrac{2}{y} &=& \dfrac{5}{6} \\\\ & \dfrac{y}{2} &=& \dfrac{6}{5} \\\\ & \mathbf{y} &=& \mathbf{\dfrac{12}{5}} \\ \hline \end{array} \\ \begin{array}{|lrcll|} \hline (1)+(2)-(3) : & \dfrac{1}{y}+\dfrac{1}{x}+ \dfrac{1}{z}+\dfrac{1}{x}-\dfrac{1}{z}-\dfrac{1}{y} &=& 1+\dfrac{1}{2}-\dfrac{1}{3} \\\\ & \dfrac{2}{x} &=& \dfrac{7}{6} \\\\ & \dfrac{x}{2} &=& \dfrac{6}{7} \\\\ & \mathbf{x} &=& \mathbf{\dfrac{12}{7}} \\ \hline \end{array} \)

 

laugh

Mar 26, 2020
 #3
avatar+129840 
+1

Thanks, EP

 

Here's another method....albeit....maybe not any easier .....LOL!!!!!

 

The  line intersects  the  y axis  at  15/4  = 3.75

And it  intersects  the  x  axis at x = -5

The distance  between these two points is   sqrt  ( 5^2 + 3.75^2 ) = 6.25

 

Call the  distance  between  the  y intercept  and the point where the line  intersects  the  upper-most part of the  circle, a

 

Call the distance  from the  x intercept of the  line  and the point where the line intersects the  "lower" part of the  circle , b

 

The  distance  between  the  y intercept and the top of the  circle =  6 -3.75 = 2.25

And the distance  from the y intercept to the  bottom part of the  circle  =  3.75 + 6 = 9.75

 

Likewise......the  distance  from  the  x intercept to the  leftmost point of the  circle  =1

And  the distance  from the  x intercept to  the  rightmost part of the  circle   =11

 

By  the intercepting  chord  theorem  we have this system

 

(a) (6.25 + b)  =  (2.25) (9.75)

(a+ 6.25) (b) =  (1) (11)            simplify

 

6.25a  + ab  =  21.9375     (1)

6.25b  + ab  =  11        (2)          subtract these

 

6.25 ( a - b)  =  10.9375

(a - b)  =  10.9375/6.25

a - b =  1.75

a = b + 1.75

 

Subbing  this  into   (2)   we get that

 

(b + 1.75 + 6.25) ( b)  =11

( b + 8) b  =11

b^2  + 8b - 11 = 0  

b^2  + 8b  = 11

b^2 + 8b + 16 = 11 + 16

(b + 4)^2  =  27        take the positive  root

b + 4 =  √27 = 3√3

b = 3√3  - 4

 

And 

 

a = b + 1.75   =   3√3  - 2.25

 

So.....the  length of  chord  AB  =  

 

b  + 6.25  + a  =

 

3√3 - 4 + 6.25  + 3√3  - 2.25   =

 

6√3  + 6.25 - 6.25  =

 

6√3

 

 

cool cool cool

Mar 26, 2020

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