A triangle is a closed, two-dimensional shape with 3 sides and 3 angles. Triangles can be classified by their sides as scalene, isosceles, or equilateral, depending on whether the sides are all different, two are equal, or all are equal. They can also be classified by their angles as acute, right, or obtuse based on whether the angles are less than, equal to, or greater than 90 degrees. The interior angles of any triangle always sum to 180 degrees, and the exterior angles always sum to 360 degrees. The perimeter is the sum of all 3 sides, and the area is equal to half the base times the height.

2. Characteristics
and Definition
Classification
by Side
In geometry…
A triangle is a closed, two-dimensional
shape. It is also a polygon.
It consists of 3 sides and 3 angles.
Classification
by Angle
Interior
Angles
Exterior
Angles
Perimeter of
a Triangle
Area of
a Triangle
Characteristics and Definition

3. Characteristics
and Definition
Classification
by Side
Scalene:
- all the three sides have different measures
Isosceles:
- two sides have equal length
Equilateral:
- all three sides are equal
Classification
by Angle
Interior
Angles
Exterior
Angles
Perimeter of
a Triangle
Area of
a Triangle
Classification by Side

4. Characteristics
and Definition
Classification
by Side
Acute:
- all of its angles are less than 90°
Right:
- one of the angles is equal to 90°
Obtuse:
- only one of its angles is more than 90°
Classification
by Angle
Interior
Angles
Exterior
Angles
Perimeter of
a Triangle
Area of
a Triangle
Classification by Angle

5. Characteristics
and Definition
Classification
by Side
All of the three interior angles of a triangle always
sum up to 180°.
Since all the interior angles of the triangle add up
to 180 degrees, every angle must be less than 180°.
Classification
by Angle
Interior
Angles
Exterior
Angles
Perimeter of
a Triangle
Area of
a Triangle
Interior Angles

6. Characteristics
and Definition
Classification
by Side
All of the three exterior angles of a triangle always
sum up to 360°.
Every exterior angle is supplementary to its
interior angle, therefore, every pair of an interior
and exterior angle must sum up to 180°.
Classification
by Angle
Interior
Angles
Exterior
Angles
Perimeter of
a Triangle
Area of
a Triangle
Exterior Angles

7. Characteristics
and Definition
Classification
by Side
The perimeter of a triangle can be defined as the
sum of all the sides of a triangle.
A triangle has three sides.
Perimeter of a Triangle = Sum of all 3 sides.
Classification
by Angle
Interior
Angles
Exterior
Angles
Perimeter of
a Triangle
Area of
a Triangle
Perimeter of a Triangle

8. Characteristics
and Definition
Classification
by Side
The area of a triangle can be defined as the half of
the product of its base and height.
Base = b
Height = h
Area of a Triangle =
𝒃𝒉
𝟐
Classification
by Angle
Interior
Angles
Exterior
Angles
Perimeter of
a Triangle
Area of
a Triangle
Area of a Triangle