Double-click on the movie to PLAY.

Click once to PAUSE.

Double-click again to return to the START of the movie.

M2+

When we rotate one of the triangles through 90˚ about D in this way,

Why do two of the sides lie on the same straight line?

Why do two of the sides exactly coincide?

Why are the areas of the triangles the same?

A more interactive version of the movie can be found HERE, in the form of a JAVA worksheet.