Find the roots of the quadratic equation x
2 – 8x = 9 by completing the square. Show your work.

Question

Find the roots of the quadratic equation x
2 – 8x = 9 by completing the square. Show your work.

kimjafo · Answer

I think its -1 and 9

wolf1728 · Answer

x^2 -8x =9

Yes it is -1 and 9 but you need to know how to do it by completing the square.

kimjafo · Answer

I believe ive figured it out

wolf1728 · Answer

x^2 -8x =9
To complete the square:
Move the non-x term to the right.
(It is already there).
Divide the equation by the coefficient of x^2 which is one so that is already done too.

Take the coefficient of 'x' , divide it by 2 then square it.
-8 divided by 2 = 
-4 and squared =
16

Add it to both sides:
x^2 -8x +16 =9 +16

Take the square root of both sides.

(x-4)^2 = 5^2

x -4 = 5
Therefore x =9 

and let's not forget that the square root of 25 also equals -5 so
x -4 = -5
x = -1