8.2 classifying triangles 1

Lesson 8.2, For use with pages 411-415

Find the complement and supplement to the angle.

1. 38º 2. 70º

Lesson 8.2, For use with pages 411-415

Complement 20ºComplement 52º

Find the complement and supplement to the angle.

1. 38º 2. 70º

Supplement 142º Supplement 110º

8.2 Classifying Triangles

Essential Questions

• Why is it important to be able to identify congruent triangles in everyday life?

• Where in real life can you use the properties of isosceles and equilateral triangles?

• How are the relationships between lines and planes used in the real world?

• What areas in the real world are properties of parallel lines important?

• Triangle: a closed figure with three sides made with line segments.

Triangles- Classifying by Angles

• Acute – 3 acute angles

• Right – has one right angle

• Obtuse – has one obtuse angle

Triangles- Classifying by Sides• Equilateral – 3 congruent sides

• Isosceles – at least 2 sides

• Scalene – no sides, no angles

• Are all equilateral triangles isosceles?• Are all isosceles equilateral?

Triangles• Triangles can also be classified by the size

of angles.– Yes – this would be a RIGHT triangle, with one

90 degree angle.

• For example:

Triangles can be classified by sides.

This is a isosceles.

• Triangles can also be classified by the size of angles.

Obtuse Triangle – named for its ONE obtuse angle. A triangle can AT MOST one obtuse angle. WHY?

Triangles can be classified by sides.

This is a scalene.

• Triangles can also be classified by the size of angles.– Acute Triangle – named for its THREE

ACUTE angles.

Triangles can be classified by sides.

This is a equilateral.

TRIANGLES

Equilateral

Scalene

Triangles

• One common characteristic of triangles is that they ALL have 180 degrees in the sum of the three angles – regardless of size and shape.

Triangles

• So if you know the measurement of 2 angles, you can always find the 3rd angle.

90° 30°

X ° 60m x °

Triangles

85 °

61 °

x °

34m x

Can the angles of a triangle have the given measures?

• 41°, 52°, 87°

• Yes, because they add up to 180°.

Can the angles of a triangle have the given measures?

• 38.2°, 49.7°, 92.2°

• No, because they add up to 180.1°.

Homework

• Page 413 #1-22, 29-31– Use pencil