Solve for x x^2-4x-21>0

$x^{2} - 4 x - 21 > 0$

Step 1

Convert the inequality to an equation.

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

Step 2

Factor $x^{2} - 4 x - 21$ using the AC method.

Consider the form $x^{2} + b x + c$ . Find a pair of integers whose product is $c$ and whose sum is $b$ . In this case, whose product is $- 21$ and whose sum is $- 4$ .

$- 7, 3$

Write the factored form using these integers.

$(x - 7) (x + 3) = 0$

Step 3

If any individual factor on the left side of the equation is equal to $0$ , the entire expression will be equal to $0$ .

$x - 7 = 0$

$x + 3 = 0$

Step 4

Set $x - 7$ equal to $0$ and solve for $x$ .

Set $x - 7$ equal to $0$ .

$x - 7 = 0$

Add $7$ to both sides of the equation.

$x = 7$

Step 5

Set $x + 3$ equal to $0$ and solve for $x$ .

Set $x + 3$ equal to $0$ .

$x + 3 = 0$

Subtract $3$ from both sides of the equation.

$x = - 3$

Step 6

The final solution is all the values that make $(x - 7) (x + 3) = 0$ true.

$x = 7, - 3$

Step 7

Use each root to create test intervals.

$x < - 3$

$- 3 < x < 7$

$x > 7$

Step 8

Choose a test value from each interval and plug this value into the original inequality to determine which intervals satisfy the inequality.

Test a value on the interval $x < - 3$ to see if it makes the inequality true.

Choose a value on the interval $x < - 3$ and see if this value makes the original inequality true.

$x = - 6$

Replace $x$ with $- 6$ in the original inequality.

${(- 6)}^{2} - 4 \cdot - 6 - 21 > 0$

The left side $39$ is greater than the right side $0$ , which means that the given statement is always true.

True

Test a value on the interval $- 3 < x < 7$ to see if it makes the inequality true.

Choose a value on the interval $- 3 < x < 7$ and see if this value makes the original inequality true.

$x = 0$

Replace $x$ with $0$ in the original inequality.

${(0)}^{2} - 4 \cdot 0 - 21 > 0$

The left side $- 21$ is not greater than the right side $0$ , which means that the given statement is false.

False

Test a value on the interval $x > 7$ to see if it makes the inequality true.

Choose a value on the interval $x > 7$ and see if this value makes the original inequality true.

$x = 10$

Replace $x$ with $10$ in the original inequality.

${(10)}^{2} - 4 \cdot 10 - 21 > 0$

The left side $39$ is greater than the right side $0$ , which means that the given statement is always true.

True

Compare the intervals to determine which ones satisfy the original inequality.

$x < - 3$ True

$- 3 < x < 7$ False

$x > 7$ True

$x < - 3$ True

$- 3 < x < 7$ False

$x > 7$ True

Step 9

The solution consists of all of the true intervals.

$x < - 3$ or $x > 7$

Step 10

The result can be shown in multiple forms.

Inequality Form:

$x < - 3 or x > 7$

Interval Notation:

$(- \infty, - 3) \cup (7, \infty)$

Step 11

Solve for x x^2-4x-21>0

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