Precalculus Examples

$\cos^{2} (x) - 2.4 \cos (x) - 0.81 = 0$

Step 1

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

${(u)}^{2} - 2.4 u - 0.81 = 0$

Step 2

Use the quadratic formula to find the solutions.

$\frac{- b \pm \sqrt{b^{2} - 4 (a c)}}{2 a}$

Step 3

Substitute the values $a = 1$ , $b = - 2.4$ , and $c = - 0.81$ into the quadratic formula and solve for $u$ .

$\frac{2.4 \pm \sqrt{{(- 2.4)}^{2} - 4 \cdot (1 \cdot - 0.81)}}{2 \cdot 1}$

Step 4

Simplify.

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Step 4.1

Simplify the numerator.

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Step 4.1.1

Raise $- 2.4$ to the power of $2$ .

$u = \frac{2.4 \pm \sqrt{5.76 - 4 \cdot 1 \cdot - 0.81}}{2 \cdot 1}$

Step 4.1.2

Multiply $- 4 \cdot 1 \cdot - 0.81$ .

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Step 4.1.2.1

Multiply $- 4$ by $1$ .

$u = \frac{2.4 \pm \sqrt{5.76 - 4 \cdot - 0.81}}{2 \cdot 1}$

Step 4.1.2.2

Multiply $- 4$ by $- 0.81$ .

$u = \frac{2.4 \pm \sqrt{5.76 + 3.24}}{2 \cdot 1}$

Step 4.1.3

Add $5.76$ and $3.24$ .

$u = \frac{2.4 \pm \sqrt{9}}{2 \cdot 1}$

Step 4.1.4

Rewrite $9$ as $3^{2}$ .

$u = \frac{2.4 \pm \sqrt{3^{2}}}{2 \cdot 1}$

Step 4.1.5

Pull terms out from under the radical, assuming positive real numbers.

$u = \frac{2.4 \pm 3}{2 \cdot 1}$

Step 4.2

Multiply $2$ by $1$ .

$u = \frac{2.4 \pm 3}{2}$

Step 5

The final answer is the combination of both solutions.

$u = 2.7, - 0.3$

Step 6

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

$\cos (x) = 2.7, - 0.3$

Step 7

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

$\cos (x) = 2.7$

$\cos (x) = - 0.3$

Step 8

Solve for $x$ in $\cos (x) = 2.7$ .

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Step 8.1

The range of cosine is $- 1 \leq y \leq 1$ . Since $2.7$ does not fall in this range, there is no solution.

No solution

Step 9

Solve for $x$ in $\cos (x) = - 0.3$ .

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Step 9.1

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

$x = \arccos (- 0.3)$

Step 9.2

Simplify the right side.

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Step 9.2.1

Evaluate $\arccos (- 0.3)$ .

$x = 1.87548898$

Step 9.3

The cosine function is negative in the second and third quadrants. To find the second solution, subtract the reference angle from $2 π$ to find the solution in the third quadrant.

$x = 2 (3.14159265) - 1.87548898$

Step 9.4

Solve for $x$ .

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Step 9.4.1

Remove parentheses.

$x = 2 (3.14159265) - 1.87548898$

Step 9.4.2

Simplify $2 (3.14159265) - 1.87548898$ .

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Step 9.4.2.1

Multiply $2$ by $3.14159265$ .

$x = 6.2831853 - 1.87548898$

Step 9.4.2.2

Subtract $1.87548898$ from $6.2831853$ .

$x = 4.40769632$

Step 9.5

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

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Step 9.5.1

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

$\frac{2 π}{| b |}$

Step 9.5.2

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

$\frac{2 π}{| 1 |}$

Step 9.5.3

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

$\frac{2 π}{1}$

Step 9.5.4

Divide $2 π$ by $1$ .

$2 π$

Step 9.6

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

$x = 1.87548898 + 2 π n, 4.40769632 + 2 π n$ , for any integer $n$

Step 10

List all of the solutions.

$x = 1.87548898 + 2 π n, 4.40769632 + 2 π n$ , for any integer $n$