Precalculus Examples

$2 \sin^{2} (x) - 7 \sin (x) - 4 = 0$

Step 1

Factor the left side of the equation.

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Step 1.1

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

$2 u^{2} - 7 u - 4 = 0$

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 $a \cdot c = 2 \cdot - 4 = - 8$ and whose sum is $b = - 7$ .

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Step 1.2.1.1

Factor $- 7$ out of $- 7 u$ .

$2 u^{2} - 7 u - 4 = 0$

Step 1.2.1.2

Rewrite $- 7$ as $1$ plus $- 8$

$2 u^{2} + (1 - 8) u - 4 = 0$

Step 1.2.1.3

Apply the distributive property.

$2 u^{2} + 1 u - 8 u - 4 = 0$

Step 1.2.1.4

Multiply $u$ by $1$ .

$2 u^{2} + u - 8 u - 4 = 0$

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.

$(2 u^{2} + u) - 8 u - 4 = 0$

Step 1.2.2.2

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

$u (2 u + 1) - 4 (2 u + 1) = 0$

Step 1.2.3

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

$(2 u + 1) (u - 4) = 0$

Step 1.3

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

$(2 \sin (x) + 1) (\sin (x) - 4) = 0$

Step 2

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

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

$\sin (x) - 4 = 0$

Step 3

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

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Step 3.1

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

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

Step 3.2

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

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Step 3.2.1

Subtract $1$ from both sides of the equation.

$2 \sin (x) = - 1$

Step 3.2.2

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

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Step 3.2.2.1

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

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

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.

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

Step 3.2.2.2.1.2

Divide $\sin (x)$ by $1$ .

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

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.

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

Step 3.2.3

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

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

Step 3.2.4

Simplify the right side.

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Step 3.2.4.1

The exact value of $\arcsin (- \frac{1}{2})$ is $- \frac{π}{6}$ .

$x = - \frac{π}{6}$

Step 3.2.5

The sine function is negative in the third and fourth quadrants. To find the second solution, subtract the solution from $2 π$ , to find a reference angle. Next, add this reference angle to $π$ to find the solution in the third quadrant.

$x = 2 π + \frac{π}{6} + π$

Step 3.2.6

Simplify the expression to find the second solution.

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Step 3.2.6.1

Subtract $2 π$ from $2 π + \frac{π}{6} + π$ .

$x = 2 π + \frac{π}{6} + π - 2 π$

Step 3.2.6.2

The resulting angle of $\frac{7 π}{6}$ is positive, less than $2 π$ , and coterminal with $2 π + \frac{π}{6} + π$ .

$x = \frac{7 π}{6}$

Step 3.2.7

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

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Step 3.2.7.1

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

$\frac{2 π}{| b |}$

Step 3.2.7.2

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

$\frac{2 π}{| 1 |}$

Step 3.2.7.3

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

$\frac{2 π}{1}$

Step 3.2.7.4

Divide $2 π$ by $1$ .

$2 π$

Step 3.2.8

Add $2 π$ to every negative angle to get positive angles.

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Step 3.2.8.1

Add $2 π$ to $- \frac{π}{6}$ to find the positive angle.

$- \frac{π}{6} + 2 π$

Step 3.2.8.2

To write $2 π$ as a fraction with a common denominator, multiply by $\frac{6}{6}$ .

$2 π \cdot \frac{6}{6} - \frac{π}{6}$

Step 3.2.8.3

Combine fractions.

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Step 3.2.8.3.1

Combine $2 π$ and $\frac{6}{6}$ .

$\frac{2 π \cdot 6}{6} - \frac{π}{6}$

Step 3.2.8.3.2

Combine the numerators over the common denominator.

$\frac{2 π \cdot 6 - π}{6}$

Step 3.2.8.4

Simplify the numerator.

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Step 3.2.8.4.1

Multiply $6$ by $2$ .

$\frac{12 π - π}{6}$

Step 3.2.8.4.2

Subtract $π$ from $12 π$ .

$\frac{11 π}{6}$

Step 3.2.8.5

List the new angles.

$x = \frac{11 π}{6}$

Step 3.2.9

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

$x = \frac{7 π}{6} + 2 π n, \frac{11 π}{6} + 2 π n$ , for any integer $n$

Step 4

Set $\sin (x) - 4$ equal to $0$ and solve for $x$ .

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Step 4.1

Set $\sin (x) - 4$ equal to $0$ .

$\sin (x) - 4 = 0$

Step 4.2

Solve $\sin (x) - 4 = 0$ for $x$ .

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Step 4.2.1

Add $4$ to both sides of the equation.

$\sin (x) = 4$

Step 4.2.2

The range of sine is $- 1 \leq y \leq 1$ . Since $4$ does not fall in this range, there is no solution.

No solution

Step 5

The final solution is all the values that make $(2 \sin (x) + 1) (\sin (x) - 4) = 0$ true.

$x = \frac{7 π}{6} + 2 π n, \frac{11 π}{6} + 2 π n$ , for any integer $n$