Two angles that are directly across from each other when two lines intersect are called vertical angles. Vertical angles are ALWAYS congruent to each other. Very good thing to know when it comes to proving triangle congruency.
If two lines intersect each other, then the vertically opposite angles are equal. Theorem 6.1 : If two lines intersect each other, then the vertically opposite angles are equal. In the statement above, it is given that 'two lines intersect each other'. So, let AB and CD be two lines intersecting at O
