HOW Are Vertical Angles Congruent?
There are 2 pairs of vertical angles formed by two intersetcing lines. By the linear pair theorem, one angle from one pair and one from the othere pair are supplementary. This second angle is supplementary to the othere angle from the first pair by the linear pairs theorem. Therefore, by the congruent supplements theorem, the first angle from the first pair of vertical angles is congruent to the second angle from the pair because they are both supplementary to the same angle.
