Chapter Three Linear Pair Theorem: If two angles from a linear pair, then they are supplementary. Vertical Angles Theorem: If two angles are vertical angles, then they have equal measures. Parallel Lines and Slopes Theorem: Two nonvertical lines are parallel if and only if they have the same slope. Transitivity of Parallelism Theorem: In a plane, if L // M and M // N, then L // N. Two Perpendiculars Theorem: If two coplanar lines L and M are each perpendicular to the same line, then they are parallel to each other. Perpendicular to parallels Theorem: Inn a plane, if a line is perpendicular to one of the two parallel lines, then it is also perpendicular to the other. Perpendicular Lines and slopes Theorem: Two nonvertical lines are perpendicular if and only if the product of their slopes is 1

