Statement

Reason

1. Let points A, B, and C form a
triangle.

given

2. Let be a line passing
through B, parallel to ,
with angles as labeled.

defining a parallel line and labeling angles

3.



4. , and
.

Congruent angles have equal measures.

5.

angle addition and definition of a straight angle

6.

substitution

What is the missing step in this proof?


Statement: , and .
Reason: Alternate Interior Angles Theorem



Statement: is parallel to .

Reason: is a transversal cutting and .



Statement: , and .

Reason: Alternate Interior Angles Theorem



Statement: , and .
Reason: and , and and are pairs of supplementary angles.