Creation of this applet was inspired by a tweet from Marilyn Burns that was retweeted by Dan Meyer. Suppose a rectangle has a length that is twice its width. A diagonal is drawn from one vertex to the opposite vertex. In addition, 2 segments are drawn from one endpoint of this diagonal to the midpoints of the sides of the rectangle not adjacent that vertex. These 3 segments cause the rectangle to be split into 4 triangles of equal area. Feel free to change the size of the rectangle by moving the BIG WHITE POINT at any time.

How do the dynamics of this applet informally suggest these 4 triangles have equal area? How can you formally prove what this applet informally illustrates?