Precalculus Examples

$f (x) = \log (\cos (x))$

Step 1

Set the argument in $\log (\cos (x))$ greater than $0$ to find where the expression is defined.

$\cos (x) > 0$

Step 2

Solve for $x$ .

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Step 2.1

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

$x > \arccos (0)$

Step 2.2

Simplify the right side.

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Step 2.2.1

The exact value of $\arccos (0)$ is $\frac{π}{2}$ .

$x > \frac{π}{2}$

Step 2.3

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.

$x = 2 π - \frac{π}{2}$

Step 2.4

Simplify $2 π - \frac{π}{2}$ .

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Step 2.4.1

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

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

Step 2.4.2

Combine fractions.

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Step 2.4.2.1

Combine $2 π$ and $\frac{2}{2}$ .

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

Step 2.4.2.2

Combine the numerators over the common denominator.

$x = \frac{2 π \cdot 2 - π}{2}$

Step 2.4.3

Simplify the numerator.

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Step 2.4.3.1

Multiply $2$ by $2$ .

$x = \frac{4 π - π}{2}$

Step 2.4.3.2

Subtract $π$ from $4 π$ .

$x = \frac{3 π}{2}$

Step 2.5

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

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Step 2.5.1

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

$\frac{2 π}{| b |}$

Step 2.5.2

Replace $b$ with $1$ in the formula for period.

$\frac{2 π}{| 1 |}$

Step 2.5.3

The absolute value is the distance between a number and zero. The distance between $0$ and $1$ is $1$ .

$\frac{2 π}{1}$

Step 2.5.4

Divide $2 π$ by $1$ .

$2 π$

Step 2.6

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

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

Step 2.7

Consolidate the answers.

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

Step 2.8

Use each root to create test intervals.

$\frac{π}{2} < x < \frac{3 π}{2}$

$\frac{3 π}{2} < x < \frac{5 π}{2}$

Step 2.9

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

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Step 2.9.1

Test a value on the interval $\frac{π}{2} < x < \frac{3 π}{2}$ to see if it makes the inequality true.

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Step 2.9.1.1

Choose a value on the interval $\frac{π}{2} < x < \frac{3 π}{2}$ and see if this value makes the original inequality true.

$x = 3$

Step 2.9.1.2

Replace $x$ with $3$ in the original inequality.

$\cos (3) > 0$

Step 2.9.1.3

The left side $- 0.98999249$ is not greater than the right side $0$ , which means that the given statement is false.

False

Step 2.9.2

Test a value on the interval $\frac{3 π}{2} < x < \frac{5 π}{2}$ to see if it makes the inequality true.

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Step 2.9.2.1

Choose a value on the interval $\frac{3 π}{2} < x < \frac{5 π}{2}$ and see if this value makes the original inequality true.

$x = 6$

Step 2.9.2.2

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

$\cos (6) > 0$

Step 2.9.2.3

The left side $0.96017028$ is greater than the right side $0$ , which means that the given statement is always true.

True

Step 2.9.3

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

$\frac{π}{2} < x < \frac{3 π}{2}$ False

$\frac{3 π}{2} < x < \frac{5 π}{2}$ True

$\frac{π}{2} < x < \frac{3 π}{2}$ False

$\frac{3 π}{2} < x < \frac{5 π}{2}$ True

Step 2.10

The solution consists of all of the true intervals.

$\frac{3 π}{2} + 2 π n < x < \frac{5 π}{2} + 2 π n$ , for any integer $n$

Step 3

The domain is all values of $x$ that make the expression defined.

Set-Builder Notation:

${x | x = \frac{3 π}{2} + 2 π n, x = \frac{5 π}{2} + 2 π n}$

Step 4