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. m21 = m 24, and
m23 = m25.
gruent angles have equal measures.
5. m 24+ m2 2 + m 25 = 180° angle addition and definition of a straight angle
6. m2 1 + m2 2 + m23 = 180° substitution
What is the missing step in this proof?
A.
Statement: 24 - 25 and 21 - 23.
Reason: Alternate Interior Angles Theorem
B.
Statement
DE
is parallel to AC
Reason: AB is a transversal cutting DE and AC
C.
Statement: 2124 and 23 - 25
Reason: Alternate Interior Angles Theorem
D.
Statement 21 24 and 2325
Reason: 21 and 24 and 23 and 25 are pairs of supplementary angles.