A property of parallelograms

We are now going to study the way to create the following figure:

First we need to define the parallelogram and its center:

A B C D parallelogram
O = barycenter(A, B, C, D)

Then, we can draw them:

draw(A, B, C, D) ; draw(O)

and the diagonals:

draw(segment(A, C), dotted)
draw(segment(B, D), dotted)

Finally, we draw the marks on the diagonals. Double dashes:

mark(segment(O, A), double)
mark(segment(O, C), double)

and crosses:

mark(segment(O, B), cross)
mark(segment(O, D), cross)