1. 13-5 The Law of Sines Warm Up Lesson Presentation Lesson Quiz
Holt Algebra 2

2. Find the area of each triangle with the given base and height.
Warm Up
Find the area of each triangle with the given base and height.
1. b =10, h = 7 2. b = 8, h = 4.6
35 units2
18.4 units2
Solve each proportion. 3. 4.
28.5 10 5.
In ∆ABC, mA = 122° and mB =17°. What is the mC ?
41°

3. Objectives
Determine the area of a triangle given side-angle-side information.
Use the Law of Sines to find the side lengths and angle measures of a triangle.

4. A sailmaker is designing a sail that will have the dimensions shown in the diagram. Based on these dimensions, the sailmaker can determine the amount of fabric needed.
The area of the triangle representing the sail is Although you do not know the value of h, you can calculate it by using the fact that sin A = , or
h = c sin A.

5. Area =
Write the area formula.
Area =
Substitute c sin A for h.
This formula allows you to determine the area of a triangle if you know the lengths of two of its sides and the measure of the angle between them.

6. An angle and the side opposite that angle are labeled with the same letter. Capital letters are used for angles, and lowercase letters are used for sides.
Helpful Hint

8. Example 1: Determining the Area of a Triangle
Find the area of the triangle. Round to the nearest tenth.
Area = ab sin C
Write the area formula.
Substitute 3 for a, 5 for b, and 40° for C.
Use a calculator to evaluate the expression. ≈
The area of the triangle is about 4.8 m2.

9. Check It Out! Example 1
Find the area of the triangle. Round to the nearest tenth.
Area = ac sin B
Write the area formula.
Substitute 8 for a, 12 for c, and 86° for B.
Use a calculator to evaluate the expression. ≈
The area of the triangle is about 47.9 m2.

10. The area of ∆ABC is equal to bc sin A or ac sin B or ab sin C
The area of ∆ABC is equal to bc sin A or ac sin B or ab sin C. By setting these expressions equal to each other, you can derive the Law of Sines.
bc sin A = ac sin B = ab sin C
Multiply each expression by 2.
bc sin A = ac sin B = ab sin C
bc sin A ac sin B ab sin C
abc =
Divide each expression by abc.
Divide out common factors.
sin A = sin B = sin C a b c

12. The Law of Sines allows you to solve a triangle as long as you know either of the following:
1. Two angle measures and any side length–angle-angle-side (AAS) or angle-side-angle (ASA) information
2. Two side lengths and the measure of an angle that is not between them–side-side-angle (SSA) information

13. Example 2A: Using the Law of Sines for AAS and ASA
Solve the triangle. Round to the nearest tenth.
Step 1. Find the third angle measure.
mD + mE + mF = 180°
Triangle Sum Theorem.
Substitute 33° for mD and 28° for mF.
33° + mE + 28° = 180°
mE = 119°
Solve for mE.

14. Example 2A Continued
Step 2 Find the unknown side lengths.
sin D
sin F d f =
sin E
sin F e f =
Law of Sines.
sin 33°
sin 28° d 15 =
sin 119°
sin 28° e 15 =
Substitute.
Cross
multiply.
d sin 28° = 15 sin 33°
e sin 28° = 15 sin 119°
e =
15 sin 119°
sin 28°
e ≈ 27.9
d =
15 sin 33°
sin 28°
d ≈ 17.4
Solve for the
unknown side.

15. Example 2B: Using the Law of Sines for AAS and ASAQ r
Solve the triangle. Round to the nearest tenth.
Step 1 Find the third angle measure.
Triangle Sum Theorem
mP = 180° – 36° – 39° = 105°

16. Example 2B: Using the Law of Sines for AAS and ASAQ r
Solve the triangle. Round to the nearest tenth.
Step 2 Find the unknown side lengths.
sin P
sin Q p q =
Law of Sines.
sin P
sin R p r =
sin 105°
sin 36° 10 q =
sin 105°
sin 39° 10 r =
Substitute.
q =
10 sin 36°
sin 105°
≈ 6.1
r =
10 sin 39°
≈ 6.5

17. Check It Out! Example 2a
Solve the triangle. Round to the nearest tenth.
Step 1 Find the third angle measure.
mH + mJ + mK = 180°
Substitute 42° for mH and 107° for mJ.
42° + 107° + mK = 180°
mK = 31°
Solve for mK.

18. Check It Out! Example 2a Continued
Step 2 Find the unknown side lengths.
sin H
sin J h j =
sin K
sin H k h =
Law of Sines.
sin 42°
sin 107° h 12 =
sin 31°
sin 42° k
8.4 =
Substitute.
Cross
multiply.
h sin 107° = 12 sin 42°
8.4 sin 31° = k sin 42°
k =
8.4 sin 31°
sin 42°
k ≈ 6.5
h =
12 sin 42°
sin 107°
h ≈ 8.4
Solve for the
unknown side.

19. Check It Out! Example 2b
Solve the triangle. Round to the nearest tenth.
Step 1 Find the third angle measure.
Triangle Sum Theorem
mN = 180° – 56° – 106° = 18°

20. Check It Out! Example 2b
Solve the triangle. Round to the nearest tenth.
Step 2 Find the unknown side lengths.
sin N
sin M n m =
Law of Sines.
sin M
sin P m p =
sin 18°
sin 106°
1.5 m =
sin 106°
sin 56°
4.7 p =
Substitute.
m =
1.5 sin 106°
sin 18°
≈ 4.7
p =
4.7 sin 56°
sin 106°
≈ 4.0

21. When you use the Law of Sines to solve a triangle for which you know side-side-angle (SSA) information, zero, one, or two triangles may be possible. For this reason, SSA is called the ambiguous case.

23. Solving a Triangle Given a, b, and mA

24. When one angle in a triangle is obtuse, the measures of the other two angles must be acute.
Remember!

25. Example 3: Art Application
Determine the number of triangular banners that can be formed using the measurements a = 50, b = 20, and mA = 28°. Then solve the triangles. Round to the nearest tenth.
Step 1 Determine the number of possible triangles. In this case, A is acute. A B C b a c
Because b < a; only one triangle is possible.

26. Example 3 Continued
Step 2 Determine mB.
Law of Sines
Substitute.
Solve for sin B.

27. Example 3 Continued
Let B represent the acute angle with a sine of Use the inverse sine function on your calculator to determine mB.
m B = Sin-1
Step 3 Find the other unknown measures of the triangle.
Solve for mC.
28° ° + mC = 180°
mC = 141.2°

28. Example 3 Continued
Solve for c.
Law of Sines
Substitute.
Solve for c.
c ≈ 66.8

29. Check It Out! Example 3
Determine the number of triangles Maggie can form using the measurements a = 10 cm, b = 6 cm, and mA =105°. Then solve the triangles. Round to the nearest tenth.
Step 1 Determine the number of possible triangles. In this case, A is obtuse.
Because b < a; only one triangle is possible.

30. Check It Out! Example 3 Continued
Step 2 Determine mB.
Law of Sines
Substitute.
Solve for sin B.
sin B ≈ 0.58

31. Check It Out! Example 3 Continued
Let B represent the acute angle with a sine of Use the inverse sine function on your calculator to determine m B.
m B = Sin-1
Step 3 Find the other unknown measures of the triangle.
Solve for mC.
105° ° + mC = 180°
mC = 39.6°

32. Check It Out! Example 3 Continued
Solve for c.
Law of Sines
Substitute.
Solve for c.
c ≈ 6.6 cm

33. Lesson Quiz: Part I
1. Find the area of the triangle. Round to the nearest tenth.
17.8 ft2
2. Solve the triangle. Round to the nearest tenth.
a  32.2; b  22.0; mC = 133.8°

34. Lesson Quiz: Part II
3. Determine the number of triangular quilt pieces that can be formed by using the measurements a = 14 cm, b = 20 cm, and mA = 39°. Solve each triangle. Round to the nearest tenth. 2;
c1  21.7 cm;
mB1 ≈ 64.0°;
mC1 ≈ 77.0°;
c2 ≈ 9.4 cm;
mB2 ≈ 116.0°;
mC2 ≈ 25.0°