Precalculus Examples

$2 \sin^{2} (x) - 9 \sin (x) + 4 = 0$

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 $\sin (x)$ .

$2 u^{2} - 9 u + 4 = 0$

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 $b = - 9$ .

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Step 1.2.1.1

Factor $- 9$ out of $- 9 u$ .

$2 u^{2} - 9 u + 4 = 0$

Step 1.2.1.2

Rewrite $- 9$ 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 1.2.2

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.

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

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

Add $1$ to 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 3.2.3

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 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.

$x = π - \frac{π}{6}$

Step 3.2.6

Simplify $π - \frac{π}{6}$ .

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Step 3.2.6.1

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

$x = π \cdot \frac{6}{6} - \frac{π}{6}$

Step 3.2.6.2

Combine fractions.

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Step 3.2.6.2.1

Combine $π$ and $\frac{6}{6}$ .

$x = \frac{π \cdot 6}{6} - \frac{π}{6}$

Step 3.2.6.2.2

Combine the numerators over the common denominator.

$x = \frac{π \cdot 6 - π}{6}$

Step 3.2.6.3

Simplify the numerator.

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Step 3.2.6.3.1

Move $6$ to the left of $π$ .

$x = \frac{6 \cdot π - π}{6}$

Step 3.2.6.3.2

Subtract $π$ from $6 π$ .

$x = \frac{5 π}{6}$

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

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

$x = \frac{π}{6} + 2 π n, \frac{5 π}{6} + 2 π n$ , 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.

$x = \frac{π}{6} + 2 π n, \frac{5 π}{6} + 2 π n$ , for any integer $n$