Solve the Inequality for x sin(x)<0

$\sin (x) < 0$

Step 1

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

$x < \arcsin (0)$

Step 2

Simplify the right side.

The exact value of $\arcsin (0)$ is $0$ .

$x < 0$

Step 3

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

$x = π - 0$

Step 4

Subtract $0$ from $π$ .

$x = π$

Step 5

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

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

$\frac{2 π}{| b |}$

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

$\frac{2 π}{| 1 |}$

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

$\frac{2 π}{1}$

Divide $2 π$ by $1$ .

$2 π$

Step 6

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

$x = 2 π n, π + 2 π n$ , for any integer $n$

Step 7

Consolidate the answers.

$x = π n$ , for any integer $n$

Step 8

Use each root to create test intervals.

$0 < x < π$

$π < x < 2 π$

Step 9

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 $0 < x < π$ to see if it makes the inequality true.

Choose a value on the interval $0 < x < π$ and see if this value makes the original inequality true.

$x = 2$

Replace $x$ with $2$ in the original inequality.

$\sin (2) < 0$

The left side $0.90929742$ is not less than the right side $0$ , which means that the given statement is false.

False

Test a value on the interval $π < x < 2 π$ to see if it makes the inequality true.

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

$x = 5$

Replace $x$ with $5$ in the original inequality.

$\sin (5) < 0$

The left side $- 0.95892427$ is less 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.

$0 < x < π$ False

$π < x < 2 π$ True

$0 < x < π$ False

$π < x < 2 π$ True

Step 10

The solution consists of all of the true intervals.

$π + 2 π n < x < 2 π + 2 π n$ , for any integer $n$

Step 11

Solve the Inequality for x sin(x)<0

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