Double-click on the movie to PLAY.
Click once to PAUSE.
RELOAD the page or double-click to return to the START of the movie.
Method B2
Here we rotate ABCD through 90˚ about M, the midpoint between D and D'.
What can we say about the resulting image and the desired image?
On the next page we choose a more or less arbitrary position for F and see what happens to the whole zig-zag line when were rotate it through 90˚ about F.
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