SOLUTION: Solve the equation by completing the square x^2 + 4x

SOLUTION: Solve the equation by completing the square x^2 + 4x - 21 = 0

Question 100954: Solve the equation by completing the square
x^2 + 4x - 21 = 0
Answer by jim_thompson5910(35256) (Show Source): You can put this solution on YOUR website!

Start with the given equation

Add 21 to both sides

Take half of the x coefficient 4 to get 2 (ie )
Now square 2 to get 4 (ie )

Add this result (4) to both sides. Now the expression is a perfect square trinomial.

Factor into (note: if you need help with factoring, check out this solver)

Combine like terms on the right side

Take the square root of both sides

Subtract 2 from both sides to isolate x.

So the expression breaks down to
or

or Take the square root of 25 to get 5

or Now combine like terms

So our answer is
or

Here is visual proof

graph of

Here we can see that the x-intercepts are and , so this verifies our answer.

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