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Method C1

Here, before rotating chord DC, we join it to the centre of rotation (the centre of the circle).
We can do this in lots of ways, but in this case we use the line DO (and we keep the angle ODC fixed so that the join is rigid).

What angle does DO (and hence chord DC) turn through? When you've run the movie, can you use the final frame to construct a proof?

On the next page, for completeness, we use the line CO to join chord DC to the centre.