Prove: the sum of the interior angle measures of labc is 180°,
statement
reason
1. let points a, b, and cform 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
4. m21 = m24, and
m23 = m25.
congruent angles have equal measures.
5. m24+ m22 - m25 = 180°
angle addition and definition of a straight angle
6. m21 - m42 - m23 = 180°
substitution
what is the missing step in this proof?