Trigonometry Examples

$x^{4} - 8 x^{2} - 9 = 0$

Step 1

Substitute $u = x^{2}$ into the equation. This will make the quadratic formula easy to use.

$u^{2} - 8 u - 9 = 0$

$u = x^{2}$

Step 2

Factor $u^{2} - 8 u - 9$ using the AC method.

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Step 2.1

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 $- 9$ and whose sum is $- 8$ .

$- 9, 1$

Step 2.2

Write the factored form using these integers.

$(u - 9) (u + 1) = 0$

Step 3

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

$u - 9 = 0$

$u + 1 = 0$

Step 4

Set $u - 9$ equal to $0$ and solve for $u$ .

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Step 4.1

Set $u - 9$ equal to $0$ .

$u - 9 = 0$

Step 4.2

Add $9$ to both sides of the equation.

$u = 9$

Step 5

Set $u + 1$ equal to $0$ and solve for $u$ .

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Step 5.1

Set $u + 1$ equal to $0$ .

$u + 1 = 0$

Step 5.2

Subtract $1$ from both sides of the equation.

$u = - 1$

Step 6

The final solution is all the values that make $(u - 9) (u + 1) = 0$ true.

$u = 9, - 1$

Step 7

Substitute the real value of $u = x^{2}$ back into the solved equation.

$x^{2} = 9$

${(x^{2})}^{1} = - 1$

Step 8

Solve the first equation for $x$ .

$x^{2} = 9$

Step 9

Solve the equation for $x$ .

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Step 9.1

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

$x = \pm \sqrt{9}$

Step 9.2

Simplify $\pm \sqrt{9}$ .

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Step 9.2.1

Rewrite $9$ as $3^{2}$ .

$x = \pm \sqrt{3^{2}}$

Step 9.2.2

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

$x = \pm 3$

Step 9.3

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

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Step 9.3.1

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

$x = 3$

Step 9.3.2

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

$x = - 3$

Step 9.3.3

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

$x = 3, - 3$

Step 10

Solve the second equation for $x$ .

${(x^{2})}^{1} = - 1$

Step 11

Solve the equation for $x$ .

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Step 11.1

Remove parentheses.

$x^{2} = - 1$

Step 11.2

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

$x = \pm \sqrt{- 1}$

Step 11.3

Rewrite $\sqrt{- 1}$ as $i$ .

$x = \pm i$

Step 11.4

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

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Step 11.4.1

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

$x = i$

Step 11.4.2

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

$x = - i$

Step 11.4.3

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

$x = i, - i$

Step 12

The solution to $x^{4} - 8 x^{2} - 9 = 0$ is $x = 3, - 3, i, - i$ .

$x = 3, - 3, i, - i$