Proof: Midpoints form Seg || to 3rd side
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In the questions below, choose the correct missing statement and reasons from the following proof.
In ΔABC, let M (not pictured) be the midpoint of ¯AB and N (not pictured) be the midpoint of ¯AC. Prove that ¯MN || ¯BC.

In ΔABC, let M (not pictured) be the midpoint of ¯AB and N (not pictured) be the midpoint of ¯AC. Prove that ¯MN || ¯BC.

Statement | Reason |
1.∠A≅∠A | 1.Reflexive Property of Congruence |
2.M is the midpoint of ¯AB | 2.Given |
3.AM=BM | 3. |
4.AM+BM=AB | 4.Segment Addition Postulate |
5.AM+AM=AB | 5.Substitution Property of Equality |
6.2AM=AB | 6.Algebra (addition) |
7.AMAB=12 | 7.Division Property of Equality |
8.N is the midpoint of ¯AC | 8.Given |
9.AN=CN | 9. |
10.AN+CN=AC | 10.Segment Addition Postulate |
11.AN+AN=AC | 11.Substitution Property of Equality |
12.2AN=AC | 12.Algebra (addition) |
13.ANAC=12 | 13.Division Property of Equality |
14.ANAC=AMAB | 14. |
15. | 15.SAS Similarity Theorem |
16.∠B≅∠AMN | 16.Coor. angles in similar triangles congruent |
17.¯BC || ¯MN | 17. |
A.
What is the missing reason in step 3?
- Definition of a midpoint
- Given
- Segment Equality Postulate
- Substitution Property of Equality
B.
What is the missing reason in step 9?
- Definition of a midpoint
- Given
- Segment Addition Postulate
- Substitution Property of Equality
C.
What is the missing reason in step 14?
- Ratios of corresponding sides in similar triangles are equal
- Division Property of Equality
- Segment Division Postulate
- Transitive Property of Equality
D.
What is the missing statement in step 15?
- ΔABC ~ ΔANM
- ΔABC ~ ΔAMN
- ΔABC ~ ΔMAN
- ΔABC ~ ΔMNA
E.
What is the missing reason in step 17?
- If two lines are cut by a transversal, and vertical angles are congruent, the two lines are parallel
- If two lines are cut by a transversal, and alternate exterior angles are congruent, the two lines are parallel
- If two lines are cut by a transversal, and corresponding angles are congruent, the two lines are parallel
- If two lines are cut by a transversal, and alternate interior angles are congruent, the two lines are parallel