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Click once to PAUSE.

RELOAD the page or double-click to return to the START of the movie.

Method A

Here we show the rotation of the red chord DC: what angle does the chord turn through?

The next page (M-B1) shows a more grounded version of the rotation by considering lines parallel to the given chords that go through the centre of the circle, ie through the centre of rotation. However, you might want to look at M-B1 (and M-B2) after first going to M-C1 - does the M-C1 movie ring true?