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Marina solved the quadratic equation $9x^2-18x-720=0$ by completing the square. In the process, she came up with the equivalent equation $$(x+r)^2 = s,$$where $r$ and $s$ are constants. What is $s$?

 May 9, 2018

Best Answer 

 #1
avatar+21869 
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Marina solved the quadratic equation $9x^2-18x-720=0$ by completing the square.

In the process, she came up with the equivalent equation $$(x+r)^2 = s,

$$where $r$ and $s$ are constants. What is $s$?

 

\(\begin{array}{|rcll|} \hline 9x^2-18x-720 &=& 0 \quad & | \quad :9 \\ x^2-2x-80 &=& 0 \\ (x-1)^2 -1-80 &=& 0 \\ (x-1)^2 &=& 81 \quad & | \quad (x-r)^2 = s \\ & & \quad & | \quad \mathbf{s=81} \\ \hline \end{array}\)

 

laugh

 May 9, 2018
edited by heureka  May 9, 2018
 #1
avatar+21869 
+2
Best Answer

Marina solved the quadratic equation $9x^2-18x-720=0$ by completing the square.

In the process, she came up with the equivalent equation $$(x+r)^2 = s,

$$where $r$ and $s$ are constants. What is $s$?

 

\(\begin{array}{|rcll|} \hline 9x^2-18x-720 &=& 0 \quad & | \quad :9 \\ x^2-2x-80 &=& 0 \\ (x-1)^2 -1-80 &=& 0 \\ (x-1)^2 &=& 81 \quad & | \quad (x-r)^2 = s \\ & & \quad & | \quad \mathbf{s=81} \\ \hline \end{array}\)

 

laugh

heureka May 9, 2018
edited by heureka  May 9, 2018

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