Vertically opposite angles

In the diagram, the lines AB and CD intersect at the point E. Click and drag any of the blue points.

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1. Name the two pairs of vertically opposite angles.
2. What relationship exists between a pair of vertically opposite angles?
3. What relationship exists between any pair of adjacent angles in the diagram?

State of New South Wales, Department of Education and Training 2007, 25/9/07, Created with GeoGebra