1. Let points A, B, and form a given
triangle
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
m_3 = m_5.
Congruent angles have equal measures.
5.m24+ m2 + m 5 = 180°
angle addition and definition of a straight angle
6. m_1 + m2 + m23 = 180°
substitution
What is the missing step in this proof?
ОА.
Statement: 24-25, and 21 - 23.
Reason: Alternate Interior Angles Theorem
OB. Statement: DE is parallel to AC
Reason: AB is a transversal cutting De and AC
Oc Statement: 21 - 24, and 3 - 25.
Reason: Alternate Interior Angles Theorem
OD Statement: 21 - 24 and 23 - 25.
Reason: 21 and 4, and 23 and 25 are pairs of supplementary angles.