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

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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

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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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#1
+6943
+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

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