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Method A

This shows what the rotation of the visible part of the zig-zag line (ie ABCD) should look like.
Can you visualise the position of the centre?

It is easy to think that the centre lies midway between a point and its image. However, this only works for 180˚ rotations. On the next page we see what happens when the point midway between D and D' is chosen as the centre.