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Routh Quadrilateral Interior Angles

Interact with this app for a few minutes. Then answer the questions that follow. Note: ALL LARGE POINTS are moveable.

Slide the "REMOVE C and D" to the right. Drag the vertices of the 4 angles so all 4 are on the right side outside of the quadrilateral. See if you can get them to coincide (lie on top of each other). Move these sectors (circle pieces) around so they don't overlap. Notice anything interesting? If so, describe. Is this true even if you change the vertices?

If you added the 4 interior angles of a quadrilateral together, what would their sum be?