Theorem: A general proposition not self-evident but proved by a chain of reasoning; a truth established by means of accepted truths.

1. Vertically opposite angles are equal in measure.

2. In an isosceles triangle the angles opposite the equal sides are equal. Conversely, if two angles are equal, then the triangle is isosceles.

3. If a transversal makes equal alternate angles on two lines then the lines are parallel. Conversely, if two lines are parallel, then any transversal will make equal alternate angles with them.

4.* The angles in any triangle add to 180.

5. Two lines are parallel if,... (More)