Trigonometry Examples

$2 \cos^{2} (2 x) - \cos (2 x) - 1 = 0$

Step 1

Factor the left side of the equation.

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Step 1.1

Let $u = \cos (2 x)$ . Substitute $u$ for all occurrences of $\cos (2 x)$ .

$2 u^{2} - u - 1 = 0$

Step 1.2

Factor by grouping.

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Step 1.2.1

For a polynomial of the form $a x^{2} + b x + c$ , rewrite the middle term as a sum of two terms whose product is $a \cdot c = 2 \cdot - 1 = - 2$ and whose sum is $b = - 1$ .

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Step 1.2.1.1

Factor $- 1$ out of $- u$ .

$2 u^{2} - (u) - 1 = 0$

Step 1.2.1.2

Rewrite $- 1$ as $1$ plus $- 2$

$2 u^{2} + (1 - 2) u - 1 = 0$

Step 1.2.1.3

Apply the distributive property.

$2 u^{2} + 1 u - 2 u - 1 = 0$

Step 1.2.1.4

Multiply $u$ by $1$ .

$2 u^{2} + u - 2 u - 1 = 0$

Step 1.2.2

Factor out the greatest common factor from each group.

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Step 1.2.2.1

Group the first two terms and the last two terms.

$(2 u^{2} + u) - 2 u - 1 = 0$

Step 1.2.2.2

Factor out the greatest common factor (GCF) from each group.

$u (2 u + 1) - (2 u + 1) = 0$

Step 1.2.3

Factor the polynomial by factoring out the greatest common factor, $2 u + 1$ .

$(2 u + 1) (u - 1) = 0$

Step 1.3

Replace all occurrences of $u$ with $\cos (2 x)$ .

$(2 \cos (2 x) + 1) (\cos (2 x) - 1) = 0$

Step 2

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

$2 \cos (2 x) + 1 = 0$

$\cos (2 x) - 1 = 0$

Step 3

Set $2 \cos (2 x) + 1$ equal to $0$ and solve for $x$ .

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Step 3.1

Set $2 \cos (2 x) + 1$ equal to $0$ .

$2 \cos (2 x) + 1 = 0$

Step 3.2

Solve $2 \cos (2 x) + 1 = 0$ for $x$ .

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Step 3.2.1

Subtract $1$ from both sides of the equation.

$2 \cos (2 x) = - 1$

Step 3.2.2

Divide each term in $2 \cos (2 x) = - 1$ by $2$ and simplify.

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Step 3.2.2.1

Divide each term in $2 \cos (2 x) = - 1$ by $2$ .

$\frac{2 \cos (2 x)}{2} = \frac{- 1}{2}$

Step 3.2.2.2

Simplify the left side.

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Step 3.2.2.2.1

Cancel the common factor of $2$ .

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Step 3.2.2.2.1.1

Cancel the common factor.

$\frac{2 \cos (2 x)}{2} = \frac{- 1}{2}$

Step 3.2.2.2.1.2

Divide $\cos (2 x)$ by $1$ .

$\cos (2 x) = \frac{- 1}{2}$

Step 3.2.2.3

Simplify the right side.

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Step 3.2.2.3.1

Move the negative in front of the fraction.

$\cos (2 x) = - \frac{1}{2}$

Step 3.2.3

Take the inverse cosine of both sides of the equation to extract $x$ from inside the cosine.

$2 x = \arccos (- \frac{1}{2})$

Step 3.2.4

Simplify the right side.

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Step 3.2.4.1

The exact value of $\arccos (- \frac{1}{2})$ is $\frac{2 π}{3}$ .

$2 x = \frac{2 π}{3}$

Step 3.2.5

Divide each term in $2 x = \frac{2 π}{3}$ by $2$ and simplify.

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Step 3.2.5.1

Divide each term in $2 x = \frac{2 π}{3}$ by $2$ .

$\frac{2 x}{2} = \frac{\frac{2 π}{3}}{2}$

Step 3.2.5.2

Simplify the left side.

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Step 3.2.5.2.1

Cancel the common factor of $2$ .

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Step 3.2.5.2.1.1

Cancel the common factor.

$\frac{2 x}{2} = \frac{\frac{2 π}{3}}{2}$

Step 3.2.5.2.1.2

Divide $x$ by $1$ .

$x = \frac{\frac{2 π}{3}}{2}$

Step 3.2.5.3

Simplify the right side.

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Step 3.2.5.3.1

Multiply the numerator by the reciprocal of the denominator.

$x = \frac{2 π}{3} \cdot \frac{1}{2}$

Step 3.2.5.3.2

Cancel the common factor of $2$ .

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Step 3.2.5.3.2.1

Factor $2$ out of $2 π$ .

$x = \frac{2 (π)}{3} \cdot \frac{1}{2}$

Step 3.2.5.3.2.2

Cancel the common factor.

$x = \frac{2 π}{3} \cdot \frac{1}{2}$

Step 3.2.5.3.2.3

Rewrite the expression.

$x = \frac{π}{3}$

Step 3.2.6

The cosine function is negative in the second and third quadrants. To find the second solution, subtract the reference angle from $2 π$ to find the solution in the third quadrant.

$2 x = 2 π - \frac{2 π}{3}$

Step 3.2.7

Solve for $x$ .

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Step 3.2.7.1

Simplify.

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Step 3.2.7.1.1

To write $2 π$ as a fraction with a common denominator, multiply by $\frac{3}{3}$ .

$2 x = 2 π \cdot \frac{3}{3} - \frac{2 π}{3}$

Step 3.2.7.1.2

Combine $2 π$ and $\frac{3}{3}$ .

$2 x = \frac{2 π \cdot 3}{3} - \frac{2 π}{3}$

Step 3.2.7.1.3

Combine the numerators over the common denominator.

$2 x = \frac{2 π \cdot 3 - 2 π}{3}$

Step 3.2.7.1.4

Multiply $3$ by $2$ .

$2 x = \frac{6 π - 2 π}{3}$

Step 3.2.7.1.5

Subtract $2 π$ from $6 π$ .

$2 x = \frac{4 π}{3}$

Step 3.2.7.2

Divide each term in $2 x = \frac{4 π}{3}$ by $2$ and simplify.

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Step 3.2.7.2.1

Divide each term in $2 x = \frac{4 π}{3}$ by $2$ .

$\frac{2 x}{2} = \frac{\frac{4 π}{3}}{2}$

Step 3.2.7.2.2

Simplify the left side.

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Step 3.2.7.2.2.1

Cancel the common factor of $2$ .

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Step 3.2.7.2.2.1.1

Cancel the common factor.

$\frac{2 x}{2} = \frac{\frac{4 π}{3}}{2}$

Step 3.2.7.2.2.1.2

Divide $x$ by $1$ .

$x = \frac{\frac{4 π}{3}}{2}$

Step 3.2.7.2.3

Simplify the right side.

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Step 3.2.7.2.3.1

Multiply the numerator by the reciprocal of the denominator.

$x = \frac{4 π}{3} \cdot \frac{1}{2}$

Step 3.2.7.2.3.2

Cancel the common factor of $2$ .

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Step 3.2.7.2.3.2.1

Factor $2$ out of $4 π$ .

$x = \frac{2 (2 π)}{3} \cdot \frac{1}{2}$

Step 3.2.7.2.3.2.2

Cancel the common factor.

$x = \frac{2 (2 π)}{3} \cdot \frac{1}{2}$

Step 3.2.7.2.3.2.3

Rewrite the expression.

$x = \frac{2 π}{3}$

Step 3.2.8

Find the period of $\cos (2 x)$ .

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Step 3.2.8.1

The period of the function can be calculated using $\frac{2 π}{| b |}$ .

$\frac{2 π}{| b |}$

Step 3.2.8.2

Replace $b$ with $2$ in the formula for period.

$\frac{2 π}{| 2 |}$

Step 3.2.8.3

The absolute value is the distance between a number and zero. The distance between $0$ and $2$ is $2$ .

$\frac{2 π}{2}$

Step 3.2.8.4

Cancel the common factor of $2$ .

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Step 3.2.8.4.1

Cancel the common factor.

$\frac{2 π}{2}$

Step 3.2.8.4.2

Divide $π$ by $1$ .

$π$

Step 3.2.9

The period of the $\cos (2 x)$ function is $π$ so values will repeat every $π$ radians in both directions.

$x = \frac{π}{3} + π n, \frac{2 π}{3} + π n$ , for any integer $n$

Step 4

Set $\cos (2 x) - 1$ equal to $0$ and solve for $x$ .

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Step 4.1

Set $\cos (2 x) - 1$ equal to $0$ .

$\cos (2 x) - 1 = 0$

Step 4.2

Solve $\cos (2 x) - 1 = 0$ for $x$ .

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Step 4.2.1

Add $1$ to both sides of the equation.

$\cos (2 x) = 1$

Step 4.2.2

Take the inverse cosine of both sides of the equation to extract $x$ from inside the cosine.

$2 x = \arccos (1)$

Step 4.2.3

Simplify the right side.

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Step 4.2.3.1

The exact value of $\arccos (1)$ is $0$ .

$2 x = 0$

Step 4.2.4

Divide each term in $2 x = 0$ by $2$ and simplify.

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Step 4.2.4.1

Divide each term in $2 x = 0$ by $2$ .

$\frac{2 x}{2} = \frac{0}{2}$

Step 4.2.4.2

Simplify the left side.

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Step 4.2.4.2.1

Cancel the common factor of $2$ .

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Step 4.2.4.2.1.1

Cancel the common factor.

$\frac{2 x}{2} = \frac{0}{2}$

Step 4.2.4.2.1.2

Divide $x$ by $1$ .

$x = \frac{0}{2}$

Step 4.2.4.3

Simplify the right side.

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Step 4.2.4.3.1

Divide $0$ by $2$ .

$x = 0$

Step 4.2.5

The cosine function is positive in the first and fourth quadrants. To find the second solution, subtract the reference angle from $2 π$ to find the solution in the fourth quadrant.

$2 x = 2 π - 0$

Step 4.2.6

Solve for $x$ .

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Step 4.2.6.1

Simplify.

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Step 4.2.6.1.1

Multiply $- 1$ by $0$ .

$2 x = 2 π + 0$

Step 4.2.6.1.2

Add $2 π$ and $0$ .

$2 x = 2 π$

Step 4.2.6.2

Divide each term in $2 x = 2 π$ by $2$ and simplify.

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Step 4.2.6.2.1

Divide each term in $2 x = 2 π$ by $2$ .

$\frac{2 x}{2} = \frac{2 π}{2}$

Step 4.2.6.2.2

Simplify the left side.

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Step 4.2.6.2.2.1

Cancel the common factor of $2$ .

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Step 4.2.6.2.2.1.1

Cancel the common factor.

$\frac{2 x}{2} = \frac{2 π}{2}$

Step 4.2.6.2.2.1.2

Divide $x$ by $1$ .

$x = \frac{2 π}{2}$

Step 4.2.6.2.3

Simplify the right side.

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Step 4.2.6.2.3.1

Cancel the common factor of $2$ .

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Step 4.2.6.2.3.1.1

Cancel the common factor.

$x = \frac{2 π}{2}$

Step 4.2.6.2.3.1.2

Divide $π$ by $1$ .

$x = π$

Step 4.2.7

Find the period of $\cos (2 x)$ .

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Step 4.2.7.1

The period of the function can be calculated using $\frac{2 π}{| b |}$ .

$\frac{2 π}{| b |}$

Step 4.2.7.2

Replace $b$ with $2$ in the formula for period.

$\frac{2 π}{| 2 |}$

Step 4.2.7.3

The absolute value is the distance between a number and zero. The distance between $0$ and $2$ is $2$ .

$\frac{2 π}{2}$

Step 4.2.7.4

Cancel the common factor of $2$ .

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Step 4.2.7.4.1

Cancel the common factor.

$\frac{2 π}{2}$

Step 4.2.7.4.2

Divide $π$ by $1$ .

$π$

Step 4.2.8

The period of the $\cos (2 x)$ function is $π$ so values will repeat every $π$ radians in both directions.

$x = π n, π + π n$ , for any integer $n$

Step 5

The final solution is all the values that make $(2 \cos (2 x) + 1) (\cos (2 x) - 1) = 0$ true.

$x = \frac{π}{3} + π n, \frac{2 π}{3} + π n, π n, π + π n$ , for any integer $n$

Step 6

Consolidate the answers.

$x = \frac{π n}{3}$ , for any integer $n$