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document.write( "We use the definition of a parabola:\r\n" );
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document.write( "A parabola is the set of points that are equidistant\r\n" );
document.write( "from the focus (a point) and the directrix (a line).\r\n" );
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document.write( "Let (x,y) be an arbitrary point on the parabola. \r\n" );
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document.write( "Essentially we set the lengths of the two green lines \r\n" );
document.write( "equal to each other. \r\n" );
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document.write( "The distance from the arbitrary point (x,y) to the \r\n" );
document.write( "focus is gotten by the formula for the distance\r\n" );
document.write( "between two given points:\r\n" );
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\r\n" );
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document.write( "The distance from the arbitrary point (x,y) to the\r\n" );
document.write( "directrix is gotten by the formula for the distance \r\n" );
document.write( "from a given point to a given line.\r\n" );
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document.write( "We write the equation of the directrix y = 2x - 9,\r\n" );
document.write( "as 2x-y-9 = 0, then the distance from the focus to \r\n" );
document.write( "the directrix is:\r\n" );
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document.write( "So we set those two distances equal to each other:\r\n" );
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document.write( "Square both sides:\r\n" );
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document.write( "That simplifies to\r\n" );
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document.write( "
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document.write( "Edwin
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document.write( "![\"\"](\"/images/stars/stars-13.gif\")
