Given: AABC Prove: The sum of the interior angle measures of AABC is 180º.
D
B
4
2
Statement
Reason
1. Let points A, B, and C form a
triangle.
given
2. Let De be a line passing
through B, parallel to AC,
with angles as labeled.
defining a parallel line and labeling angles
3.
4.m_1 = m_4, and
m23 = m_5.
Congruent angles have equal measures.
5.m24+ m2 + m25 = 180°
angle addition and definition of a straight angle
6. m 1 + m 2 + m 3 = 180°
substitution
What is the missing step in this proof?
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