Solve by Completing the Square x^2+4x-21=0

$x^{2} + 4 x - 21 = 0$

Step 1

Add $21$ to both sides of the equation.

$x^{2} + 4 x = 21$

Step 2

To create a trinomial square on the left side of the equation, find a value that is equal to the square of half of $b$ .

${(\frac{b}{2})}^{2} = {(2)}^{2}$

Step 3

Add the term to each side of the equation.

$x^{2} + 4 x + {(2)}^{2} = 21 + {(2)}^{2}$

Step 4

Simplify the equation.

Simplify the left side.

Raise $2$ to the power of $2$ .

$x^{2} + 4 x + 4 = 21 + {(2)}^{2}$

Simplify the right side.

Simplify $21 + {(2)}^{2}$ .

Raise $2$ to the power of $2$ .

$x^{2} + 4 x + 4 = 21 + 4$

Add $21$ and $4$ .

$x^{2} + 4 x + 4 = 25$

Step 5

Factor the perfect trinomial square into ${(x + 2)}^{2}$ .

${(x + 2)}^{2} = 25$

Step 6

Solve the equation for $x$ .

Take the specified root of both sides of the equation to eliminate the exponent on the left side.

$x + 2 = \pm \sqrt{25}$

Simplify $\pm \sqrt{25}$ .

Rewrite $25$ as $5^{2}$ .

$x + 2 = \pm \sqrt{5^{2}}$

Pull terms out from under the radical, assuming positive real numbers.

$x + 2 = \pm 5$

The complete solution is the result of both the positive and negative portions of the solution.

First, use the positive value of the $\pm$ to find the first solution.

$x + 2 = 5$

Move all terms not containing $x$ to the right side of the equation.

Subtract $2$ from both sides of the equation.

$x = 5 - 2$

Subtract $2$ from $5$ .

$x = 3$

Next, use the negative value of the $\pm$ to find the second solution.

$x + 2 = - 5$

Move all terms not containing $x$ to the right side of the equation.

Subtract $2$ from both sides of the equation.

$x = - 5 - 2$

Subtract $2$ from $- 5$ .

$x = - 7$

The complete solution is the result of both the positive and negative portions of the solution.

$x = 3, - 7$

Solve by Completing the Square x^2+4x-21=0

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