Ellipse From String and Tacks

Question: [i]Given the ellipse equation, x²/a² + y²/b²=1 How can I show that any two rays drawn from the foci and meeting at a point on the arc add up to the same length?[/i] I ask the question this way: [i]Given two tacks (foci) and a string (constant length), pull the string tight and draw. Prove that the resulting figure has the algebraic equation x²/a² + y²/b²=1.[/i] ______________________ Accompanying text: [url]http://mathosaurus.blogspot.com/2013/06/constructing-ellipse.html[/url] The equation at the top of the applet is an intermediate step.

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