How to solve this quadratic equation with the quadratic formula?

Question

I've been having trouble with this problem:

x2 + 2x = 8

So far this is what I got:

-2±√-28
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     2
 
However I'm not too sure what to do next, help?

Answer 1

Chris,

 

To use quadratic formula, you need all the terms to be on one side of the equation:

x²+2x=8,

x²+2x-8=0.

a=1, b=2, c=-8

 

x₁=(-2+√(2²-4•1•(-8))/(2•1)

x₁=(-2+√36)/2

x₁=3

 

Try to calculate x₂ using expression (-2-√(2²-4•1•(-8))/(2•1).

Answer 2

The correct answer is [-2±√(4-(-8)) / 2 which gives {-4,2} as the solution set.

 

You need to bring the right-side constant over to the left side (it now becomes negative 8) and that makes the quantity under the square root 4+32, or 6 when the square root is taken.

 

You can also solve this by factoring, after setting the entire quadratic to zero.