Cut-the-Knot-Action (3)!
Creation of this applet was inspired by a tweet from Alexander Bogomolny (cut-the-knot.org).
Note:
This applet does not work perfectly under all circumstances.
It works best if the quadrilateral (to be formed) remains convex and if point K lies outside the quadrilateral.
How can you formally prove what this applet informally illustrates?