If two lines are cut by a transversal, then corresponding angles are congruent.

If two lines are parallel, then all angles are congruent.

If two lines are cut by a transversal, then alternate interior angles are congruent.

If two parallel lines are cut by a transversal, then alternate interior angles are congruent.

Vertical angles are congruent.

Corresponding angles are always equal.

If two parallel lines are cut by a transversal, corresponding angles are congruent.

Alternate interior angles are congruent.

It is already proven by other theorems.

It is not important.

It is assumed to be true.

It is too complex to prove.

Use the symmetric property.

Apply the transitive property.

Show angle three is congruent to angle two.

Prove angle three is congruent to angle six directly.

Angle one and angle four

Angle two and angle three

Angle three and angle six

Angle two and angle six