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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?

AnonymousConfusedGuy  Oct 22, 2017

Best Answer 

 #1
avatar+7068 
+1

9x2 - 18x - 720  =  0       Divide through by  9 .

 

x2 - 2x - 80  =  0             Add  80  to both sides.

 

x2 - 2x  =  80                  Add  (2/2)2  , which is  1 ,  to both sides of the equation.

 

x2 - 2x + 1  =  80 + 1      Factor the left side as a perfect square trinomial.

 

(x - 1)(x - 1)  =  81

 

(x - 1)2  =  81

 

Now we have it in the form  (x + r)2  =  s  , and we can see that  s = 81 .

hectictar  Oct 22, 2017
 #1
avatar+7068 
+1
Best Answer

9x2 - 18x - 720  =  0       Divide through by  9 .

 

x2 - 2x - 80  =  0             Add  80  to both sides.

 

x2 - 2x  =  80                  Add  (2/2)2  , which is  1 ,  to both sides of the equation.

 

x2 - 2x + 1  =  80 + 1      Factor the left side as a perfect square trinomial.

 

(x - 1)(x - 1)  =  81

 

(x - 1)2  =  81

 

Now we have it in the form  (x + r)2  =  s  , and we can see that  s = 81 .

hectictar  Oct 22, 2017

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