Newton's theorem (quadrilateral)

The set of Newton, named after Isaac Newton (1643-1727), is a statement in elementary geometry that states that the center of the inscribed circle of a tangent quadrilateral lies on the line connecting the two center points of its diagonals.

Let ABCD be a tangent quadrilateral with at most one pair of parallel sides and diagonals AC and BD. Further examples are E and F, the center points of the diagonals, then the center of the inscribed circle P is located on the connecting line of E and F, the so-called Newton - line.