3. Classifying Triangles by Sides
If all 3 sides are
congruent - Equilateral
If no sides are If at least 2 sides are
congruent - congruent - isosceles
Scalene
4. Classifying Triangles by Angles
All angles < 90°,
then ACUTE triangle
One angle > 90°,
then OBTUSE triangle
RIGHT triangle
if one angle = 90°
5. Now you try…
Classify each triangle by sides and angles
A.
B.
C.
7. Vocab continued…
Interior angle
Angle formed on the interior of the triangle
Exterior angle
Angle formed on the outside of the triangle
A
exterior angle Interior angle
C
B
8. Triangle Sum Theorem
The sum of the interior angles of a triangle
is 180°
9. Exterior Angle Theorem
The measure of an exterior angle of a
triangle is equal to the sum of the two
nonadjacent interior angles.
Example: m∠1=m∠A+ m∠B
B
1
A
C
10. Now you try…
Find the measure of each angle (hint: what is
their sum?)
2x + 10
x x+2
11. Practice B
Given that ∠ A is 40º and
∠B is 28º, what is the measure of
∠BCD?
A
C D
What is the measure of ∠ACB?
12. Vocabulary for Right Triangles
Legs
The sides that form the right angle
Hypotenuse
The side that is opposite the right angle
Hypotenuse
Legs
13. Corollary to the Triangle Sum Theorem
In a right triangle, the the acute angles are
complementary.
14. Now you try…
A. Using the triangle below, what
is the length of the hypotenuse?
B. What about the lengths of the
legs?
C. If one of the acute angles has a
13
measure of 35 , give the
measure of the other two 12
angles.
5
15. Vocabulary for Isosceles
Triangles
Legs
The two sides that are congruent
Base
The side that isn’t a leg
Base Angles
The angles formed at each end of the base, which
are congruent
Vertex Angle
The angle formed by the legs
