Precalculus Examples

$a = 10$ , $b = 9$ , $A = 30 °$

Step 1

The law of sines is based on the proportionality of sides and angles in triangles. The law states that for the angles of a non-right triangle, each angle of the triangle has the same ratio of angle measure to sine value.

$\frac{\sin (A)}{a} = \frac{\sin (B)}{b} = \frac{\sin (C)}{c}$

Step 2

Substitute the known values into the law of sines to find $B$ .

$\frac{\sin (B)}{9} = \frac{\sin (30 °)}{10}$

Step 3

Solve the equation for $B$ .

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Step 3.1

Multiply both sides of the equation by $9$ .

$9 \frac{\sin (B)}{9} = 9 \frac{\sin (30 °)}{10}$

Step 3.2

Simplify both sides of the equation.

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Step 3.2.1

Simplify the left side.

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Step 3.2.1.1

Cancel the common factor of $9$ .

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Step 3.2.1.1.1

Cancel the common factor.

$9 \frac{\sin (B)}{9} = 9 \frac{\sin (30 °)}{10}$

Step 3.2.1.1.2

Rewrite the expression.

$\sin (B) = 9 \frac{\sin (30 °)}{10}$

Step 3.2.2

Simplify the right side.

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Step 3.2.2.1

Simplify $9 \frac{\sin (30 °)}{10}$ .

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Step 3.2.2.1.1

The exact value of $\sin (30 °)$ is $\frac{1}{2}$ .

$\sin (B) = 9 \frac{\frac{1}{2}}{10}$

Step 3.2.2.1.2

Multiply the numerator by the reciprocal of the denominator.

$\sin (B) = 9 (\frac{1}{2} \cdot \frac{1}{10})$

Step 3.2.2.1.3

Multiply $\frac{1}{2} \cdot \frac{1}{10}$ .

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Step 3.2.2.1.3.1

Multiply $\frac{1}{2}$ by $\frac{1}{10}$ .

$\sin (B) = 9 \frac{1}{2 \cdot 10}$

Step 3.2.2.1.3.2

Multiply $2$ by $10$ .

$\sin (B) = 9 (\frac{1}{20})$

Step 3.2.2.1.4

Combine $9$ and $\frac{1}{20}$ .

$\sin (B) = \frac{9}{20}$

Step 3.3

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

$B = \arcsin (\frac{9}{20})$

Step 3.4

Simplify the right side.

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Step 3.4.1

Evaluate $\arcsin (\frac{9}{20})$ .

$B = 26.74368395$

Step 3.5

The sine function is positive in the first and second quadrants. To find the second solution, subtract the reference angle from $180$ to find the solution in the second quadrant.

$B = 180 - 26.74368395$

Step 3.6

Subtract $26.74368395$ from $180$ .

$B = 153.25631604$

Step 3.7

The solution to the equation $B = 26.74368395$ .

$B = 26.74368395, 153.25631604$

Step 3.8

Exclude the invalid angle.

$B = 26.74368395$

Step 4

The sum of all the angles in a triangle is $180$ degrees.

$30 ° + C + 26.74368395 = 180$

Step 5

Solve the equation for $C$ .

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Step 5.1

Add $30 °$ and $26.74368395$ .

$C + 56.74368395 = 180$

Step 5.2

Move all terms not containing $C$ to the right side of the equation.

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Step 5.2.1

Subtract $56.74368395$ from both sides of the equation.

$C = 180 - 56.74368395$

Step 5.2.2

Subtract $56.74368395$ from $180$ .

$C = 123.25631604$

Step 6

$\frac{\sin (A)}{a} = \frac{\sin (B)}{b} = \frac{\sin (C)}{c}$

Step 7

Substitute the known values into the law of sines to find $c$ .

$\frac{\sin (123.25631604)}{c} = \frac{\sin (30 °)}{10}$

Step 8

Solve the equation for $c$ .

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Step 8.1

Factor each term.

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Step 8.1.1

Evaluate $\sin (123.25631604)$ .

$\frac{0.8362257}{c} = \frac{\sin (30 °)}{10}$

Step 8.1.2

The exact value of $\sin (30 °)$ is $\frac{1}{2}$ .

$\frac{0.8362257}{c} = \frac{\frac{1}{2}}{10}$

Step 8.1.3

Multiply the numerator by the reciprocal of the denominator.

$\frac{0.8362257}{c} = \frac{1}{2} \cdot \frac{1}{10}$

Step 8.1.4

Multiply $\frac{1}{2} \cdot \frac{1}{10}$ .

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Step 8.1.4.1

Multiply $\frac{1}{2}$ by $\frac{1}{10}$ .

$\frac{0.8362257}{c} = \frac{1}{2 \cdot 10}$

Step 8.1.4.2

Multiply $2$ by $10$ .

$\frac{0.8362257}{c} = \frac{1}{20}$

Step 8.2

Find the LCD of the terms in the equation.

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Step 8.2.1

Finding the LCD of a list of values is the same as finding the LCM of the denominators of those values.

$c, 20$

Step 8.2.2

Since $c, 20$ contains both numbers and variables, there are two steps to find the LCM. Find LCM for the numeric part $1, 20$ then find LCM for the variable part $c^{1}$ .

Step 8.2.3

The LCM is the smallest positive number that all of the numbers divide into evenly.

1. List the prime factors of each number.

2. Multiply each factor the greatest number of times it occurs in either number.

Step 8.2.4

The number $1$ is not a prime number because it only has one positive factor, which is itself.

Not prime

Step 8.2.5

The prime factors for $20$ are $2 \cdot 2 \cdot 5$ .

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Step 8.2.5.1

$20$ has factors of $2$ and $10$ .

$2 \cdot 10$

Step 8.2.5.2

$10$ has factors of $2$ and $5$ .

$2 \cdot 2 \cdot 5$

Step 8.2.6

Multiply $2 \cdot 2 \cdot 5$ .

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Step 8.2.6.1

Multiply $2$ by $2$ .

$4 \cdot 5$

Step 8.2.6.2

Multiply $4$ by $5$ .

$20$

Step 8.2.7

The factor for $c^{1}$ is $c$ itself.

$c^{1} = c$

$c$ occurs $1$ time.

Step 8.2.8

The LCM of $c^{1}$ is the result of multiplying all prime factors the greatest number of times they occur in either term.

$c$

Step 8.2.9

The LCM for $c, 20$ is the numeric part $20$ multiplied by the variable part.

$20 c$

Step 8.3

Multiply each term in $\frac{0.8362257}{c} = \frac{1}{20}$ by $20 c$ to eliminate the fractions.

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Step 8.3.1

Multiply each term in $\frac{0.8362257}{c} = \frac{1}{20}$ by $20 c$ .

$\frac{0.8362257}{c} (20 c) = \frac{1}{20} (20 c)$

Step 8.3.2

Simplify the left side.

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Step 8.3.2.1

Rewrite using the commutative property of multiplication.

$20 \frac{0.8362257}{c} c = \frac{1}{20} (20 c)$

Step 8.3.2.2

Multiply $20 \frac{0.8362257}{c}$ .

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Step 8.3.2.2.1

Combine $20$ and $\frac{0.8362257}{c}$ .

$\frac{20 \cdot 0.8362257}{c} c = \frac{1}{20} (20 c)$

Step 8.3.2.2.2

Multiply $20$ by $0.8362257$ .

$\frac{16.72451418}{c} c = \frac{1}{20} (20 c)$

Step 8.3.2.3

Cancel the common factor of $c$ .

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Step 8.3.2.3.1

Cancel the common factor.

$\frac{16.72451418}{c} c = \frac{1}{20} (20 c)$

Step 8.3.2.3.2

Rewrite the expression.

$16.72451418 = \frac{1}{20} (20 c)$

Step 8.3.3

Simplify the right side.

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Step 8.3.3.1

Cancel the common factor of $20$ .

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Step 8.3.3.1.1

Factor $20$ out of $20 c$ .

$16.72451418 = \frac{1}{20} (20 (c))$

Step 8.3.3.1.2

Cancel the common factor.

$16.72451418 = \frac{1}{20} (20 c)$

Step 8.3.3.1.3

Rewrite the expression.

$16.72451418 = c$

Step 8.4

Rewrite the equation as $c = 16.72451418$ .

$c = 16.72451418$

Step 9

These are the results for all angles and sides for the given triangle.

$A = 30 °$

$B = 26.74368395$

$C = 123.25631604$

$a = 10$

$b = 9$

$c = 16.72451418$