Precalculus Examples

$\sin (2 x) = 2 \sin (x)$

Step 1

Subtract $2 \sin (x)$ from both sides of the equation.

$\sin (2 x) - 2 \sin (x) = 0$

Step 2

Apply the sine double-angle identity.

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

Step 3

Factor $2 \sin (x)$ out of $2 \sin (x) \cos (x) - 2 \sin (x)$ .

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Step 3.1

Factor $2 \sin (x)$ out of $- 2 \sin (x)$ .

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

Step 3.2

Factor $2 \sin (x)$ out of $2 \sin (x) \cos (x) + 2 \sin (x) \cdot - 1$ .

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

Step 4

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

$\sin (x) = 0$

$\cos (x) - 1 = 0$

Step 5

Set $\sin (x)$ equal to $0$ and solve for $x$ .

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Step 5.1

Set $\sin (x)$ equal to $0$ .

$\sin (x) = 0$

Step 5.2

Solve $\sin (x) = 0$ for $x$ .

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Step 5.2.1

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

$x = \arcsin (0)$

Step 5.2.2

Simplify the right side.

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Step 5.2.2.1

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

$x = 0$

Step 5.2.3

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 5.2.4

Subtract $0$ from $π$ .

$x = π$

Step 5.2.5

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

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Step 5.2.5.1

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

$\frac{2 π}{| b |}$

Step 5.2.5.2

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

$\frac{2 π}{| 1 |}$

Step 5.2.5.3

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

$\frac{2 π}{1}$

Step 5.2.5.4

Divide $2 π$ by $1$ .

$2 π$

Step 5.2.6

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 6

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

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Step 6.1

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

$\cos (x) - 1 = 0$

Step 6.2

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

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Step 6.2.1

Add $1$ to both sides of the equation.

$\cos (x) = 1$

Step 6.2.2

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

$x = \arccos (1)$

Step 6.2.3

Simplify the right side.

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Step 6.2.3.1

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

$x = 0$

Step 6.2.4

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.

$x = 2 π - 0$

Step 6.2.5

Subtract $0$ from $2 π$ .

$x = 2 π$

Step 6.2.6

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

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Step 6.2.6.1

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

$\frac{2 π}{| b |}$

Step 6.2.6.2

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

$\frac{2 π}{| 1 |}$

Step 6.2.6.3

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

$\frac{2 π}{1}$

Step 6.2.6.4

Divide $2 π$ by $1$ .

$2 π$

Step 6.2.7

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

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

Step 7

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

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

Step 8

Consolidate the answers.

$x = π n$ , for any integer $n$

Step 9