1. δabc and δdef are similar = given
2. bc/ef = ab/de = property of similar triangles
3. ab/bc = ec/de = property of proportion
4. slope of ac = bc/ab = definition of slope
5. slope of df = ef/de = definition of slope
6. = substitution property of equality

the table contains the proof of the theorem of the relationship between slopes of parallel lines. what is the missing statement for step 6?

a. slope of ac = -1/slope of df
b. slope of ac = 1/slope of df
c. slope of ac = -slope of df
d. slope of ac = slope of df
e. slope of ac = 1 -slope of df