01--Quadrilaterals and their Diagonals
A Quibble about Problem 6
The theorem that if the opposite sides of a quadrilateral are equal, then it is a parallelogram does not seem to exclude the possibility
that the figure might cross over itself. Drag Points B or C around
in a circle to see this effect. Change the proportions of the figure
by dragging the sliders.
To rule out this interpretation, we should make it clear that the edges of a polygon meet only the adjacent edges, and only at the endpoints.
A Quibble about Prob 6
Quadrilaterals with Equal Diagonals
If a quadrilateral is a rectangle, the diagonals are equal.
The converse is not true. Move the yellow control points.
