Solve by Completing the Square x^2+10x+21=0

$x^{2} + 10 x + 21 = 0$

Step 1

Subtract $21$ from both sides of the equation.

$x^{2} + 10 x = - 21$

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 $b$ .

${(\frac{b}{2})}^{2} = {(5)}^{2}$

Step 3

Add the term to each side of the equation.

$x^{2} + 10 x + {(5)}^{2} = - 21 + {(5)}^{2}$

Step 4

Simplify the equation.

Simplify the left side.

Raise $5$ to the power of $2$ .

$x^{2} + 10 x + 25 = - 21 + {(5)}^{2}$

Simplify the right side.

Simplify $- 21 + {(5)}^{2}$ .

Raise $5$ to the power of $2$ .

$x^{2} + 10 x + 25 = - 21 + 25$

Add $- 21$ and $25$ .

$x^{2} + 10 x + 25 = 4$

Step 5

Factor the perfect trinomial square into ${(x + 5)}^{2}$ .

${(x + 5)}^{2} = 4$

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.

$x + 5 = \pm \sqrt{4}$

Simplify $\pm \sqrt{4}$ .

Rewrite $4$ as $2^{2}$ .

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

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

$x + 5 = \pm 2$

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.

$x + 5 = 2$

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

Subtract $5$ from both sides of the equation.

$x = 2 - 5$

Subtract $5$ from $2$ .

$x = - 3$

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

$x + 5 = - 2$

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

Subtract $5$ from both sides of the equation.

$x = - 2 - 5$

Subtract $5$ from $- 2$ .

$x = - 7$

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+10x+21=0

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