[color=#000000]This applet follows this applet entitled [url=https://www.geogebra.org/m/WM8gVzJ6]Japanese Theorem for Cyclic Quadrilaterals[/url]. [br][br]What principle is dynamically being illustrated here? [br]The 4 circles are incircles of the 4 different triangles you can form from this cyclic quadrilateral. [br][br]How can you formally prove what this applet informally illustrates?[br][/color][br]