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Method A2

In this movie F moves beyond D to C and then all the way back to A.

We can use the movie to reinforce the conjecture that FG is parallel to DE. Or, if we have already proved this, we can focus on what happens as G approaches B: what happens to the cyclic quadrilateral GBCF and the exterior angle AGF ? We shift from a configuration involving a theorem about the angles of a cyclic quadrilateral to a theorem about the angle between a tangent and chord.

In the movie on the next page F moves in a more extreme way still: beyond C and then back beyond A.