Precalculus Examples

$7 - 11 \sin (x) = 2 \cos^{2} (x)$

Step 1

Subtract $2 \cos^{2} (x)$ from both sides of the equation.

$7 - 11 \sin (x) - 2 \cos^{2} (x) = 0$

Step 2

Replace the $- 2 \cos^{2} (x)$ with $- 2 (1 - \sin^{2} (x))$ based on the $\sin^{2} (x) + \cos^{2} (x) = 1$ identity.

$7 - 11 \sin (x) - 2 (1 - \sin^{2} (x)) = 0$

Step 3

Simplify each term.

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Step 3.1

Apply the distributive property.

$7 - 11 \sin (x) - 2 \cdot 1 - 2 (- \sin^{2} (x)) = 0$

Step 3.2

Multiply $- 2$ by $1$ .

$7 - 11 \sin (x) - 2 - 2 (- \sin^{2} (x)) = 0$

Step 3.3

Multiply $- 1$ by $- 2$ .

$7 - 11 \sin (x) - 2 + 2 \sin^{2} (x) = 0$

Step 4

Subtract $2$ from $7$ .

$- 11 \sin (x) + 5 + 2 \sin^{2} (x) = 0$

Step 5

Reorder the polynomial.

$2 \sin^{2} (x) - 11 \sin (x) + 5 = 0$

Step 6

Substitute $u$ for $\sin (x)$ .

$2 {(u)}^{2} - 11 u + 5 = 0$

Step 7

Factor by grouping.

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Step 7.1

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 $a \cdot c = 2 \cdot 5 = 10$ and whose sum is $b = - 11$ .

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Step 7.1.1

Factor $- 11$ out of $- 11 u$ .

$2 u^{2} - 11 u + 5 = 0$

Step 7.1.2

Rewrite $- 11$ as $- 1$ plus $- 10$

$2 u^{2} + (- 1 - 10) u + 5 = 0$

Step 7.1.3

Apply the distributive property.

$2 u^{2} - 1 u - 10 u + 5 = 0$

Step 7.2

Factor out the greatest common factor from each group.

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Step 7.2.1

Group the first two terms and the last two terms.

$2 u^{2} - 1 u - 10 u + 5 = 0$

Step 7.2.2

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

$u (2 u - 1) - 5 (2 u - 1) = 0$

Step 7.3

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

$(2 u - 1) (u - 5) = 0$

Step 8

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

$2 u - 1 = 0$

$u - 5 = 0$

Step 9

Set $2 u - 1$ equal to $0$ and solve for $u$ .

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Step 9.1

Set $2 u - 1$ equal to $0$ .

$2 u - 1 = 0$

Step 9.2

Solve $2 u - 1 = 0$ for $u$ .

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Step 9.2.1

Add $1$ to both sides of the equation.

$2 u = 1$

Step 9.2.2

Divide each term in $2 u = 1$ by $2$ and simplify.

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Step 9.2.2.1

Divide each term in $2 u = 1$ by $2$ .

$\frac{2 u}{2} = \frac{1}{2}$

Step 9.2.2.2

Simplify the left side.

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Step 9.2.2.2.1

Cancel the common factor of $2$ .

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Step 9.2.2.2.1.1

Cancel the common factor.

$\frac{2 u}{2} = \frac{1}{2}$

Step 9.2.2.2.1.2

Divide $u$ by $1$ .

$u = \frac{1}{2}$

Step 10

Set $u - 5$ equal to $0$ and solve for $u$ .

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Step 10.1

Set $u - 5$ equal to $0$ .

$u - 5 = 0$

Step 10.2

Add $5$ to both sides of the equation.

$u = 5$

Step 11

The final solution is all the values that make $(2 u - 1) (u - 5) = 0$ true.

$u = \frac{1}{2}, 5$

Step 12

Substitute $\sin (x)$ for $u$ .

$\sin (x) = \frac{1}{2}, 5$

Step 13

Set up each of the solutions to solve for $x$ .

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

$\sin (x) = 5$

Step 14

Solve for $x$ in $\sin (x) = \frac{1}{2}$ .

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Step 14.1

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

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

Step 14.2

Simplify the right side.

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Step 14.2.1

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

$x = \frac{π}{6}$

Step 14.3

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.

$x = π - \frac{π}{6}$

Step 14.4

Simplify $π - \frac{π}{6}$ .

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Step 14.4.1

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 14.4.2

Combine fractions.

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Step 14.4.2.1

Combine $π$ and $\frac{6}{6}$ .

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

Step 14.4.2.2

Combine the numerators over the common denominator.

$x = \frac{π \cdot 6 - π}{6}$

Step 14.4.3

Simplify the numerator.

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Step 14.4.3.1

Move $6$ to the left of $π$ .

$x = \frac{6 \cdot π - π}{6}$

Step 14.4.3.2

Subtract $π$ from $6 π$ .

$x = \frac{5 π}{6}$

Step 14.5

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

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Step 14.5.1

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

$\frac{2 π}{| b |}$

Step 14.5.2

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

$\frac{2 π}{| 1 |}$

Step 14.5.3

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

$\frac{2 π}{1}$

Step 14.5.4

Divide $2 π$ by $1$ .

$2 π$

Step 14.6

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 15

Solve for $x$ in $\sin (x) = 5$ .

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Step 15.1

The range of sine is $- 1 \leq y \leq 1$ . Since $5$ does not fall in this range, there is no solution.

No solution

Step 16

List all of the solutions.

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