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

2. Triangles classified by sides
A scalene triangle
has no congruent sides.
An isosceles triangle
has at least 2 congruent sides
An equilateral triangle, all of the sides are congruent.

3. Triangles classified by angles
In an acute triangle, all of the angles are acute.
An obtuse triangle has
one obtuse angle.
A right triangle
has one right angle.

4. Classify each triangle according to its sides and angles.
EX1)
isosceles
Two congruent sides
acute
Three acute angles
This is an isosceles acute triangle.
EX2)
scalene
No congruent sides
right
One right angle
This is a scalene right triangle.

5. EX3)
equilateral
Three congruent sides
acute
Three acute angles
EX4)
scalene
No congruent sides
right
One right angle
EX5)
isosceles
Two congruent sides
obtuse
One obtuse angle
EX6)
isosceles
Two congruent sides
right
One right angle

6. Tear off the corners of your triangle and put them together.
The sum of the measures of the angles in a triangle is 180°. 2
m1 + m2 + m3 = 180° 1 3

7. Find the measure of the unknown angle.
EX7)
55°
80° x
The sum of the measures of the angles is 180°.
80° + 55° + x = 180°
135° + x = 180°
–135°
–135°
Subtract 135° from both sides.
x = 45°
The measure of the unknown angle is 45°.

8. The sum of the measures of the angles is 180°. 90° + 30° + x = 180°
EX8)
30°
90° x
The sum of the measures of the angles is 180°.
90° + 30° + x = 180°
120° + x = 180°
Combine like terms.
–120°
–120°
Subtract 120° from both sides.
x = 60°
The measure of the unknown angle is 60°.

9. notes

10. Date ______________
Classify Triangles
Classify triangles by the lengths of their sides and their angles.

11. Triangles classified by sides
A scalene triangle
An isosceles triangle
An equilateral triangle,

12. Triangles classified by angles
In an acute triangle
An obtuse triangle has
A right triangle

13. Classify each triangle according to its sides and angles.
EX1)
EX2)

14. EX3)
EX4)
EX5)
EX6)

15. Find the measure of the unknown angle.
EX7)
55°
80° x

16. EX8)
30°
90° x