Proving the Sum of the Interior Angle Measures of a Triangle Is 180°
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Given: yll z
Prove: m25+ m2 2 + m26 = 180°
Angles Lines Statements Reasons
given
def. of
substitution
def. of straight angle
A
M
1
2 3
Y
Reasons
2. angle addition postulate
5
6
7 z
Statements
✓ 2. m 21+ m2 2 + m23 =
mZLAM
✓ 3. m21+ m2 2 + m23 = 180
4. 21-25
5. 23-26
6. mZ1 = m25
✓ 7. m3 = m26
✓ 8. m25+ m2 2 + m26 = 180
3. def. of straight angle
4. alternate interior angles theorem
5. alternate interior angles theorem
6. def. of
7. def. of
С
B
Correct! You have completed this
exercise
8. substitution