Theorem 01 - Vertically Opposite Angles
Vertically Opposite Angles occur whenever two lines or line segments intersect. Vertically opposite angles are always equal. In the diagram below, the red angles are vertically opposite, as are the blue angles.
Click and drag the points A and B. Notice that the angles change size but the opposite angles are always equal. Notice that the adjacent angles always add to 180 degrees. Such angles are called supplementary angles.