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.