Sum of Angles in Triangles

Triangles classified by sides

A scalene trianglehas no congruent sides.

An isosceles trianglehas at least 2 congruent sides

An equilateral triangle, all of the sides are congruent.

Triangles classified by angles

In an acute triangle, all of the angles are acute.

An obtuse triangle hasone obtuse angle.

A right triangle has one right angle.

Classify each triangle according to its sides and angles.

EX1) Two congruent sides

acute Three acute angles

This is an acute isosceles triangle.

EX2)No congruent sides

right One right angle

This is a right scalene triangle.

isosceles

scalene

EX3) Three congruent sides

acute Three acute angles

EX4) No congruent sides

right One right angle

equilateral

scalene

EX5) Two congruent sides

obtuse One obtuse angle

EX6)Two congruent sides

right One right angle

isosceles

isosceles

Tear off the corners of your triangle and put them together.

The sum of the measures of the angles in a triangle is 180°.

m1 + m2 + m3 = 180° 1 3

2

Find the measure of the unknown angle.

55°

80° x

80° + 55° + x = 180°

135° + x = 180°–135° –135°

x = 45°

The sum of the measures of the angles is 180°.

EX7)

90° + 2x + x = 180° 90° + 3x = 180°

–90° –90°

3x = 90°

90°

x

2x

EX8)

3 = 3

x = 30°2x = 60°

AssignmentPage 6131-16

notes

Classify Triangles

Classify triangles by the lengths of their sides and their angles.

Date ______________

Triangles classified by sides

A scalene triangle

An isosceles triangle

An equilateral triangle,

Triangles classified by angles

In an acute triangle

An obtuse triangle has

A right triangle

Classify each triangle according to its sides and angles.

EX1)

EX2)

EX3)

EX4)

EX5)

EX6)

Find the measure of the unknown angle.

55°

80° x

EX7)

90°

30°

x

EX8)