Question

1. ad ≅ bc; ad || bc 1. given
2. ∠cad and ∠acb are alternate interior ∠s 2. definition of alternate interior angles
3. ∠cad ≅ ∠acb 3. alternate interior angles are congruent
4. ac ≅ ac 4. reflexive property
5. △cad ≅ △acb 5. sas congruency theorem
6. ab ≅ cd 6.?
7. abcd is a parallelogram 7. parallelogram side theorem

what is the missing reason in step 6?
sss congruency theorem
cpctc
definition of a parallelogram

Explanation:

CPCTC (Corresponding - Parts of Congruent Triangles are Congruent) is used when we have already proven two triangles congruent (in step 5, $\triangle CAD\cong\triangle ACB$) and we want to show that corresponding sides (in this case $\overline{AB}\cong\overline{CD}$) are congruent.

Answer:

CPCTC