Question 220849: solve the problem by completing the square.
-x2+4x+8=0
Answer by likaaka(51)   (Show Source):
You can put this solution on YOUR website! -x^2+4x+8=0, first move the loose number to the other side
-x^2+4x=-8, divide thru by the coefficient of the squared term in this case a -1 (basically you need the coefficient of the squared term to be 1)
(-x^2)/(-1) + (4x)/(-1) = (-8)/(-1), simplify the equation
x^2-4x=8, take 1/2 the coefficient of the x-term and square it & be sure to take the sign
1/2 of -4 is -2 (this number, -2, will be used later)and (-2)^2 is 4, now add this number to both sides of the equation
x^2-4x +4=8 +4, convert the left-hand side to squared form and simplify the right hand side (this is where you use the sign that you took note of earlier and insert it into the middle of the parenthetical part
(x-2)^2=12
Now you must solve for x, start by taking the square root of both sides
x-2= + or - the square root of 12
the square root of 12 simplifies to 2 root 3 or 2 times the square root of 3
now add two to both sides
x= + or -2 root 3 +2
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