MGSE8.G.5

​Standard: MGSE8.G.5

Use informal arguments to establish facts about the angle sum and exterior angle of triangles, about the angles created when parallel lines are cut by a transversal, and the angle-angle criterion for similarity of triangles. For example, arrange three copies of the same triangle so that the sum of the three angles appears to form a line, and give an argument in terms of transversals why this is so.

8th Grade Math | MGSE8.G.5

​Learning Targets

R1: I can reason and explain why the angle-sum and exterior-angle theorems of a triangle are true.

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8th Grade Math | MGSE8.G.5

​Vocabulary

​Triangle - a plane figure with three straight sides and three angles.

Triangle sum theorem - The sum of the measures of the interior angles of a triangle is 180°.

​Exterior angle theorem - An exterior angle of a triangle is equal to the sum of its two opposite non-adjacent interior angles. The sum of the exterior angle and the adjacent interior angle that is not opposite is equal to 180º.

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8th Grade Math | MGSE8.G.5

​Guiding Questions

​What is a triangle?

What is the ​triangle sum theorem?

What is ​exterior angle theorem?

How do you use your knowledge of the triangle sum theorem and exterior angle theorem to figure out interior and exterior angle measurements?​

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8th Grade Math | MGSE8.G.5

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Multiple Choice

What does tri mean?

• two

• three

• one

• five

Multiple Choice

A a plane figure with three straight sides and three angles.

• triangle

• square

• rectangle

• circle

Multiple Choice

To classify triangles by the measure of their sides you must use the names:

• Equilateral, Equiangular and Obtuse

• Equilateral, Isosceles and Scalene

• Right triangle, Scalene and Equiangular

• Isosceles, Scalene and Equilateral

Multiple Choice

To classify triangles by the measure of their angles we use the names:

• Equiangular, Isosceles and Acute

• Acute, Obtuse and Isosceles

• Right triangle, Isosceles triangle and Scalene triangle

• Right triangle, Obtuse triangle and Acute triangle

Multiple Choice

When the triangle has one right angle it is called:

• Acute triangle

• Obtuse triangle

• Right triangle

• Scalene triangle

Multiple Choice

When a triangle has an obtuse angle inside it is called:

• Right triangle

• Obtuse triangle

• Acute triangle

• Isosceles triangle

Multiple Choice

If a triangle has three acute angles inside it is called:

• Acute triangle

• Obtuse triangle

• Right triangle

• Isosceles

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Multiple Choice

How many degrees is X?

• 65

• 70

• 90

• 55

Multiple Choice

How many degrees is X?

• 37

• 47

• 7

• 90

Multiple Choice

How much is the value of X?

• 85

• 95

• 75

• 100

Multiple Choice

How much is the value of X?

• 90

• 45

• 30

• 1

Multiple Choice

How much is the value of X?

• 45

• 60

• 90

• 15

Poll

Do you understand today's lesson?

• Yes, I totally got this!

• I have no idea how to do this.

• I need to practice a little to understand better.