Solve for x cos(2x)=1/2

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Trigonometry

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

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

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

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

$2 x = \frac{π}{3}$

Divide each term by $2$ and simplify.

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

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

Cancel the common factor of $2$ .

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Cancel the common factor.

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

Divide $x$ by $1$ .

$x = \frac{π}{3} \cdot \frac{1}{2}$

Multiply $\frac{π}{3} \cdot \frac{1}{2}$ .

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Multiply $\frac{π}{3}$ and $\frac{1}{2}$ .

$x = \frac{π}{3 \cdot 2}$

Multiply $3$ by $2$ .

$x = \frac{π}{6}$

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.

$2 x = 2 π - \frac{π}{3}$

Simplify the expression to find the second solution.

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To write $\frac{2 π}{1}$ as a fraction with a common denominator, multiply by $\frac{3}{3}$ .

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

Write each expression with a common denominator of $3$ , by multiplying each by an appropriate factor of $1$ .

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Combine.

$\frac{2 π \cdot 3}{1 \cdot 3} - \frac{π}{3}$

Multiply $3$ by $1$ .

$\frac{2 π \cdot 3}{3} - \frac{π}{3}$

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

Combine the numerators over the common denominator.

$2 x = \frac{2 π \cdot 3 - π}{3}$

Multiply $3$ by $2$ .

$2 x = \frac{6 π - π}{3}$

Subtract $π$ from $6 π$ .

$2 x = \frac{5 π}{3}$

Divide each term by $2$ and simplify.

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Divide each term in $2 x = \frac{5 π}{3}$ by $2$ .

$\frac{2 x}{2} = \frac{5 π}{3} \cdot \frac{1}{2}$

Cancel the common factor of $2$ .

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Cancel the common factor.

$\frac{2 x}{2} = \frac{5 π}{3} \cdot \frac{1}{2}$

Divide $x$ by $1$ .

$x = \frac{5 π}{3} \cdot \frac{1}{2}$

Multiply $\frac{5 π}{3} \cdot \frac{1}{2}$ .

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Multiply $\frac{5 π}{3}$ and $\frac{1}{2}$ .

$x = \frac{5 π}{3 \cdot 2}$

Multiply $3$ by $2$ .

$x = \frac{5 π}{6}$

Find the period.

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

$\frac{2 π}{| b |}$

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

$\frac{2 π}{| 2 |}$

Solve the equation.

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

$\frac{2 π}{2}$

Cancel the common factor of $2$ .

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Cancel the common factor.

$\frac{2 π}{2}$

Divide $π$ by $1$ .

$π$

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

$x = \frac{π}{6} + π n, \frac{5 π}{6} + π n$ , for any integer $n$

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