6.8 The Triangle-Sum Property


Use the Triangle-Sum Property to find
the measures of angles.
Explain the consequences of the
Triangle-Sum Property
Name:_______________________________
BIG IDEA: In any triangle, the sum of the measures of the three angles is 180⁰.
I will be able to __________________________________________________________________
_______________________________________________________________________________
Warm Up
Draw a triangle on a piece of paper. Repeat what Kayo’s grandfather did by cutting off the
corners of the triangle and placing the vertices of the angles together. Did you get the same
result? _____________________________
What is the sum of the measures of the three angles?_________________________
Vocabulary
Definition
Deductive reasoning or
deduction
To reason logically from
some things that you
assume true or know to be
true.
Triangle-Sum Property
 The sum of the measures of the three angles of any triangle is 180⁰
Question:
What is the sum of the angles of an isosceles triangle? _______________
Example 1
Suppose two angles of a triangle measure 36⁰ and 64⁰. What is the measure of the third angle?
______________________
Example 2
Find the measures of ∟TOP and ∟TSO .
Example 3
Find the measures of ∟DAC in the drawing.
Example 4
Use the information given in the drawing. Explain the steps needed to find m∟9.
_______________________________________________________________________________
_________________________________________________________________________________
Notes:
6. 8 Exit Ticket
Name:
Refer to the figure at the right.
1. Find the m∟BAC ________________
2. Find the m∟DAB ________________
3. What is the sum of the m∟B +m∟C?_________________
4. What is the relationship between the m∟B +m∟C and m∟DAB?
___________________________________________________________________________
5. __________Which is True?
a. In some but not all triangles, the sum of the measures of the angles is 180⁰.
b. In all triangles, the sum of the measures of the all triangles is 180⁰.
c. The sum of the measures of the angles of a triangle can be any number form 180⁰ to
360