SOLUTION: The problem: 2x^2 - 12x - 7 = 0 the directions are to solve by completing the square. This is what I did: 2x^2 - 12x = 7 2x^2 - 12x + 36 = 43 2(x(x-6) + 18) = 43 x(x-6)+ 18 =

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Question 771122: The problem: 2x^2 - 12x - 7 = 0
the directions are to solve by completing the square.
This is what I did:
2x^2 - 12x = 7
2x^2 - 12x + 36 = 43
2(x(x-6) + 18) = 43
x(x-6)+ 18 = (43/2)
x(x-6) = (7/2)
x^2 - 6x = (7/2)
I keep getting stuck
I know that the answers are 6.5355 and -.5355 from plugging it into the quadratic formula, but I don't know how to get there by completing the square.
Thank you so much for any help
Answer by Edwin McCravy(20055)   (Show Source): You can put this solution on YOUR website!
2x² - 12x = 7

You didn't divide through by the coefficient of x².
You must do that first:

 = 

x² - 6x = 

Half of -6 is -3.  Square -3, (-3)² = 9,  Add 9 to both sides:

x² - 6x + 9 =  + 9

(x-3)(x-3) = 

(x - 3)² = 

√(x - 3)² = 

x - 3 = 

x - 3 = 

x - 3 = 

x - 3 = 

x - 3 = 

    x = 3  ±  

    x =   ± 

    x = 

That's the correct answer.

2x^2 - 12x = 7
2x^2 - 12x + 36 = 43 <--- forgot to divide through by 2
2(x(x-6) + 18) = 43 <--- shouldn't factor x out of the first two terms
x(x-6)+ 18 = (43/2)
x(x-6) = (7/2)
x^2 - 6x = (7/2)
Edwin
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