Given: ΔABC Prove: The sum of the interior angle measures of ΔABC is 180°.

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. m∠1 = m∠4, and
m∠3 = m∠5.

Congruent angles have equal measures.

5. m∠4 + m∠2 + m∠5 = 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?