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Ellipse: Reflective Property

In the applet below, a tangent line is drawn to a point on an ellipse with foci F1 and F2.  Interact with the following applet for a few minutes. As you do, be sure to change the locations of the LARGE POINTS each time before re-sliding the slider. Please answer the questions that appear below the applet.


How would you describe what you see here in your own words?


How can we formally prove what is dynamically illustrated here?

Quick (Silent) Demo