1. Warm Up
Problem of the Day
Lesson Presentation
Lesson Quizzes

2. Warm Up
Solve each equation.
x + 37 = 180
2. x = 180
3. 2x + 18 = 180
= 2x x
x = 81
x = 79
x = 81
x = 36

3. Problem of the Day
What is the one hundred fiftieth day of a non-leap year?
May 30

4. Learn to find unknown angles and identify possible side lengths in triangles.

5. Vocabulary Triangle Sum Theorem acute triangle right triangle
obtuse triangle
equilateral triangle
isosceles triangle
scalene triangle
Triangle Inequality Theorem

6. If you tear off two corners of a triangle and place them next to the third corner, the three angles seem to form a straight line.

7. Draw a triangle and extend one side
Draw a triangle and extend one side. Then draw a line parallel to the extended side, as shown.
The sides of the triangle are transversals to the parallel lines.
The three angles in the triangle can be arranged to form a straight line or 180°.

8. An acute triangle has 3 acute angles
An acute triangle has 3 acute angles. A right triangle has 1 right angle. An obtuse triangle has 1 obtuse angle.

9. Additional Example 1A: Finding Angles in Acute, Right, and Obtuse Triangles
Find c° in the right triangle.
42° + 90° + c° = 180°
132° + c° = 180°
–132° –132°
c° = 48°

10. Additional Example 1B: Finding Angles in Acute, Right, and Obtuse Triangles
Find m° in the obtuse triangle.
23° + 62° + m° = 180°
85° + m° = 180°
–85° –85°
m° = 95°

11. Additional Example 1C: Finding Angles in Acute, Right and Obtuse Triangles
Find p° in the acute triangle.
73° + 44° + p° = 180°
117° + p° = 180°
–117° –117°
p° = 63°

12. Check It Out: Example 1A
Find b in the right triangle.
38°
38° + 90° + b° = 180°
128° + b° = 180°
–128° –128°
b° = 52° b°

13. Check It Out: Example 1B
Find a° in the acute triangle.
88° + 38° + a° = 180°
38°
126° + a° = 180°
–126° –126°
a° = 54° a°
88°

14. Check It Out: Example 1C
Find c° in the obtuse triangle.
24° + 38° + c° = 180°
38°
62° + c° = 180°
24° c°
–62° –62°
c° = 118°

15. An equilateral triangle has 3 congruent sides and 3 congruent angles
An equilateral triangle has 3 congruent sides and 3 congruent angles. An isosceles triangle has at least 2 congruent sides and 2 congruent angles. A scalene triangle has no congruent sides and no congruent angles.

16. Additional Example 2A: Finding Angles in Equilateral, Isosceles, and Scalene Triangles
Find the angle measures in the isosceles triangle.
62° + t° + t° = 180°
Triangle Sum Theorem
62° + 2t° = 180°
Combine like terms.
–62° –62°
Subtract 62° from both sides.
2t° = 118°
2t° = 118°
Divide both sides by 2.
t° = 59°
The angles labeled t° measure 59°.

17. Find the angle measures in the scalene triangle.
Additional Example 2B: Finding Angles in Equilateral, Isosceles, and Scalene Triangles
Find the angle measures in the scalene triangle.
2x° + 3x° + 5x° = 180°
Triangle Sum Theorem
10x° = 180°
Combine like terms.
Divide both sides by 10.
x = 18°
The angle labeled 2x° measures 2(18°) = 36°, the angle labeled 3x° measures 3(18°) = 54°, and the angle labeled 5x° measures 5(18°) = 90°.

18. Additional Example 2C: Finding Angles in Equilateral, Isosceles, and Scalene Triangles
Find the angle measures in the equilateral triangle.
3b° = 180°
Triangle Sum Theorem
3b° ° =
Divide both sides by 3.
b° = 60°
All three angles measure 60°.

19. Check It Out: Example 2A
Find the angle measures in the isosceles triangle.
39° + t° + t° = 180°
Triangle Sum Theorem
39° + 2t° = 180°
Combine like terms.
–39° –39°
Subtract 39° from both sides.
2t° = 141°
2t° = 141°
Divide both sides by 2
39°
t° = 70.5° t°
The angles labeled t° measure 70.5°. t°

20. Find the angle measures in the scalene triangle.
Check It Out: Example 2B
Find the angle measures in the scalene triangle.
3x° + 7x° + 10x° = 180°
Triangle Sum Theorem
20x° = 180°
Combine like terms.
Divide both sides by 20.
x = 9°
10x°
The angle labeled 3x° measures 3(9°) = 27°, the angle labeled 7x° measures 7(9°) = 63°, and the angle labeled 10x° measures 10(9°) = 90°.
3x°
7x°

21. Check It Out: Example 2C
Find the angle measures in the equilateral triangle.
3x° = 180°
Triangle Sum Theorem
3x° ° = x°
x° = 60° x° x°
All three angles measure 60°.

22. Additional Example 3: Finding Angles in a Triangle that Meets Given Conditions
The second angle in a triangle is six times as large as the first. The third angle is half as large as the second. Find the angle measures and draw a possible picture.
Let x° = the first angle measure. Then 6x° = second angle measure, and (6x°) = 3x° = third angle measure. 12

23. Additional Example 3 Continued
Let x° = the first angle measure. Then 6x° = second angle measure, and (6x°) = 3x° = third angle. 12
x° + 6x° + 3x° = 180°
Triangle Sum Theorem
10x° = 180°
Combine like terms.
Divide both sides by 10.
x° = 18°

24. Additional Example 3 Continued
Let x° = the first angle measure. Then 6x° = second angle measure, and (6x°) = 3x° = third angle. 12
x° = 18°
The angles measure 18°, 54°, and 108°. The triangle is an obtuse scalene triangle.
3 • 18° = 54°
6 • 18° = 108°

25. Check It Out: Example 3
The second angle in a triangle is three times larger than the first. The third angle is one third as large as the second. Find the angle measures and draw a possible figure.
Let x° = the first angle measure. Then 3x° = second angle measure, and (3x°) = x° = third angle measures. 13

26. Check It Out: Example 3 Continued
Let x° = the first angle measure. Then 3x° = second angle measure, and (3x°) = 3x° = third angle. 13
x° + 3x° + x° = 180°
Triangle Sum Theorem
5x° = 180°
Combine like terms.
Divide both sides by 5.
x° = 36°

27. Check It Out: Example 3 Continued
Let x° = the first angle measure. Then 3x° = second angle measure, and (3x°) = x° = third angle. 13
The angles measure 36°, 36°, and 108°. The triangle is an obtuse isosceles triangle.
x° = 36°
3 • 36° = 108°
x° = 36°
36°
108°

28. Additional Example 4A: Using the Triangle Inequality Theorem
Tell whether a triangle can have sides with the given lengths. Explain.
8 ft, 10 ft, 13 ft
Find the sum of the lengths of each pair of sides and compare it to the third side. ? ? ?
> 13
> 13
> 10
18 > 13 
23 > 13 
21 > 10 
A triangle can have these side lengths. The sum of the lengths of any two sides is greater than the length of the third side.

29. Additional Example 4B: Using the Triangle Inequality Theorem
Tell whether a triangle can have sides with the given lengths. Explain.
2 m, 4 m, 6 m
Find the sum of the lengths of each pair of sides and compare it to the third side. ?
2 + 4 > 6
6 > 6 
A triangle cannot have these side lengths. The sum of the lengths of two sides is not greater than the length of the third side.

30. Check It Out: Example 4
Tell whether a triangle can have sides with the given lengths. Explain.
17 m, 15 m, 33 m
Find the sum of the lengths of each pair of sides and compare it to the third side. ?
> 33
32 > 33 
A triangle cannot have these side lengths. The sum of the lengths of two sides is not greater than the length of the third side.

31. Lesson Quiz for Student Response Systems
Lesson Quizzes
Standard Lesson Quiz
Lesson Quiz for Student Response Systems 31

32. Lesson Quiz: Part I
1. Find the missing angle measure in the acute triangle shown.
38°
2. Find the missing angle measure in the right triangle shown.
55°

33. Lesson Quiz: Part II
3. Find the missing angle measure in an acute triangle with angle measures of 67° and 63°.
50°
4. Tell whether a triangle can have sides with lengths of 4 cm, 8 cm, and 12 cm.
No; is not greater than 12

34. Lesson Quiz for Student Response Systems
1. Identify the missing angle measure in the acute triangle shown.
A. 43°
B. 57°
C. 80°
D. 90° 34

35. Lesson Quiz for Student Response Systems
2. Identify the missing angle measure in the acute triangle shown.
A. 40°
B. 50°
C. 90°
D. 180° 35

36. Lesson Quiz for Student Response Systems
3. Identify the missing angle measure in an acute triangle with angle measures of 38° and 61°.
A. 38°
B. 61°
C. 81°
D. 99° 36