Sum of interior angles of a triangle

Geometry

Angles of a Triangle

Objective: Outcome E3 – Make and apply
generalizations about the sum of the angles in
triangles and quadrilaterals.

Sum of the
angles of a

triangle
a

c

b

Angles of a Triangle

a

c

b

 Draw any triangle on a piece

of paper.

 Mark the corners a, b and c.

 Tear the triangle into three

corners.

Angles of a Triangle

 Draw any triangle on a piece

of paper.

 Mark the corners a, b and c.

 Tear the triangle into three

corners.
a

c

b

Angles of a Triangle

 Arrange the three pieces so

the corners are adjoining.

 What do you notice?

a

c

b

Angles of a Triangle

 What do you notice?

 What kind of angle to they form?

 What is the measure of the

angle?

a

c

b

Angles of a Triangle

 The sum of the

angles of any triangle

equals 180°.

a

c

b
a + b + c = 180°

Finding Angles of a Triangle

 If we know two angles of
a triangle, we can find the
third angle by subtracting

the first two from 180°.

80°

40°
?

180
- 80
- 40

?

? =

Finding Angles of a Triangle

 If we know two angles of
a triangle, we can find the
third angle by subtracting

the first two from 180°.

80°

40°
?

180
- 80
- 40

60

? = 60°

Finding Angles of a Triangle

 Find the measure of the

unknown angle.

100°

35°

?

Finding Angles of a Triangle

 Find the measure of the

unknown angle.

50°

75°

?