Section 3.3:Parallel Lines and Triangle Sum TheoremObjectives:•Classify triangles and find the measures of their angles•Use the exterior angles of triangles

The sum of the measures of the angles of a triangle is 180 degrees

Triangle Angle Sum Theorem

180 CmBmAmA

BC

Find the m ∠ 1.

1.

2.

1

36°

1

15°

17°

Using Algebra: Find the values of x, y, and z.

65° z°y°x°

21°39°

Find the measure of each numbered angle:

1.

2.

55°

21

3

45°

78

40°

9

Find the value of the variable:

x°

(2x)°

Classifying Triangles by its Angles Equiangular: all ∠’s are ≅

Acute: all ∠’s are less than 90°

Right: One right angle

Obtuse: One angle greater than 90° (but less than 180°

Equilateral: All sides are ≅

Isosceles: At least 2 sides ≅

Scalene: No ≅ sides

Classifying Triangles by Sides

Triangle Exterior Angle Theorem

Exterior ∠ of a Polygon:Formed by a side and an extension of a side

Remote Interior ∠:Non-adjacent, interior ∠’s

Triangle Exterior Angle Theorem:m∠ACD = m∠A + m∠ B

Find the indicated measures:

1.

2.

125°

1

128.5°13°

2

Find the measure of each numbered angle. Justify your answer.

1.

2.

75° 2 1 3

48°

78°

2 3

40°