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help Me please.

 

System of equations: Solve the system of equation

 

{xy=8}

{x2+y2=20}

ManuelBautista2019  Mar 6, 2018

Best Answer 

 #2
avatar+7340 
+3

xy  =  8      Divide both sides of this equation by  y .

x  =  8/y     Use this value for  x  in the second given equation.

 

x2 + y2  =  20

(8/y)2 + y2   =   20

64/y2 + y2   =   20           Multiply through by  y2

64 + y4  =  20y2              Subtract  20y2  from both sides.

y4 - 20y2 + 64  =  0         We can rewrite this as...

(y2)2 - 20y2 + 64  =  0     Factor like this:   u2 - 20u + 64  =   (u - 4)(u - 16)

(y2 - 4)(y2 - 16)  =  0       Factor each difference of squares.

 

(y + 2)(y - 2)(y + 4)(y - 4)  =  0      Set each factor equal to zero.

 

y + 2  =  0     or     y - 2  =  0     or     y + 4  =  0     or     y - 4  =  0

y  =  -2          or     y  =  2          or     y  =  -4          or     y  =  4

 

Find  x  at each of these values for  y .

 

When  y  =  -2  ,     x  =  8 / y  =  8 / -2  =  -4

 

When  y  =  2  ,     x  =  8 / y  =  8 / 2  =  4

 

When  y  =  -4  ,     x  =  8 / y  =  8 / -4  =  -2

 

When  y  =  4  ,     x  =  8 / y  =  8 / 4  =  2

 

The solution set is   { (-2, -4), (2, 4), (-4, -2), (4, 2) }

hectictar  Mar 6, 2018
 #1
avatar+11 
-1

answering will be pointless he's not on

ClownPrinceofChaos  Mar 6, 2018
 #2
avatar+7340 
+3
Best Answer

xy  =  8      Divide both sides of this equation by  y .

x  =  8/y     Use this value for  x  in the second given equation.

 

x2 + y2  =  20

(8/y)2 + y2   =   20

64/y2 + y2   =   20           Multiply through by  y2

64 + y4  =  20y2              Subtract  20y2  from both sides.

y4 - 20y2 + 64  =  0         We can rewrite this as...

(y2)2 - 20y2 + 64  =  0     Factor like this:   u2 - 20u + 64  =   (u - 4)(u - 16)

(y2 - 4)(y2 - 16)  =  0       Factor each difference of squares.

 

(y + 2)(y - 2)(y + 4)(y - 4)  =  0      Set each factor equal to zero.

 

y + 2  =  0     or     y - 2  =  0     or     y + 4  =  0     or     y - 4  =  0

y  =  -2          or     y  =  2          or     y  =  -4          or     y  =  4

 

Find  x  at each of these values for  y .

 

When  y  =  -2  ,     x  =  8 / y  =  8 / -2  =  -4

 

When  y  =  2  ,     x  =  8 / y  =  8 / 2  =  4

 

When  y  =  -4  ,     x  =  8 / y  =  8 / -4  =  -2

 

When  y  =  4  ,     x  =  8 / y  =  8 / 4  =  2

 

The solution set is   { (-2, -4), (2, 4), (-4, -2), (4, 2) }

hectictar  Mar 6, 2018
 #3
avatar+20705 
+1

System of equations: Solve the system of equation

 

{xy=8}

{x2+y2=20}

 

Formula:

\(\begin{array}{|rcll|} \hline (x+y)^2 &=& x^2+2xy+y^2 \\ x^2+y^2 &=& (x+y)^2-2xy \quad & | \quad xy = 8 \\ x^2+y^2 &=& (x+y)^2 - 8 \\ \hline \end{array}\)

 

1.

\(\begin{array}{|rcll|} \hline x^2+y^2 &=& 20 \quad & | \quad x^2+y^2 = (x+y)^2-16 \\ (x+y)^2-16 &=& 20 \\ (x+y)^2 &=& 20+16 \\ (x+y)^2 &=& 36 \\ \mathbf{x+y} &\mathbf{=}& \mathbf{\pm 6} \\ \hline \end{array}\)

 

Two Systems of equation now:

\(\begin{array}{|lrcl|lrcl|} \hline (1)& xy &=& 8 &(1) & xy &=& 8 \\ &\mathbf{y} &\mathbf{=}& \mathbf{\frac{8}{x}} & & \mathbf{y} &\mathbf{=}& \mathbf{\frac{8}{x}} \\\\ (2)& x+y &=& 6 &(2)& x+y &=& -6 \\ & x+\frac{8}{x} &=& 6 & & x+\frac{8}{x} &=& -6 \\ & x^2+8 &=& 6x & & x^2+8 &=& -6x \\ & x^2-6x+8 &=& 0 & & x^2+6x+8 &=& 0 \\ & x_{1,2}&=& \frac{6\pm \sqrt{36-4\cdot 8} }{2} & & x_{3,4}&=& \frac{-6\pm \sqrt{36-4\cdot 8} }{2} \\ & x_{1,2}&=& \frac{6\pm 4 }{2} & & x_{3,4}&=& \frac{-6\pm 4 }{2} \\ & x_{1,2}&=& 3\pm 1 & & x_{3,4}&=& -3\pm 1 \\\\ & \mathbf{x_1 =4} && \mathbf{x_2 = 2} & & \mathbf{x_3 = -2} && \mathbf{x_4 =-4} \\ & \mathbf{y_1} =\frac{8}{4}\mathbf{=2} && \mathbf{y_2} = \frac{8}{2}\mathbf{=4} & & \mathbf{y_3} = \frac{8}{-2}\mathbf{=-4} && \mathbf{y_4} = \frac{8}{-4}\mathbf{=-2} \\ \hline \end{array} \)

 

 

laugh

heureka  Mar 7, 2018

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