400
Prove the alternate angle theorem from the corresponding angles conjecture. (the diagram is set up as such: there are two parallel lines a and b which are cut by a transversal named c, on line a, the vertical pair angles are called 1 and 2, on the same side of c and 1 and congruent to angle one is the angle 3)
1. a parallel b with transversal c (given), 2. angle1 congruent angle3 (corresponding angle postulate0, 3. angle1 congruent angle2 (vertical angles congruent), 4. angle2 congruent angle3 (transitive property) QED