Verified answer

Working backwards, we see that proving that corresponding angles B and 4 to be congruent would allow us to then conclude that ED is parallel to AB. We already know angles 1 and 4 are congruent, so now it is enough to prove angles 1 and B are congruent; this is easy to do because we know triangle ABC is isosceles with AC = BC.

Now we are ready to write the formal two-column proof. Note: the = signs in the proof mean congruence.

1. AC = BC; angle 1 = angle 4 1. Given

2. angle B = angle 1 2. If two sides of a triangle are congruent, so are the angles opposite the sides.

3. angle B = angle 4 3. Transitive property of congruence

4. ED || AB 4. If two lines cut by a transversal form congruent corresponding angles, the lines are parallel.

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