| 1. | The dashed blue line is the directrix of the ellipse shown.
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| 2. | Conversely, two tangents which intersect on the directrix are perpendicular.
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| 3. | One directrix and its focus is enough to produce both arms of the hyperbola.
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| 4. | But for the other conics it does, you have to use the directrix.
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| 5. | Also, the polar line of a focus of the conic is its corresponding directrix.
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| 6. | Neither Dandelin nor Quetelet used the Dandelin spheres to prove the focus-directrix property.
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| 7. | The directrix'circle'becomes a curve with zero curvature, indistinguishable from a straight line.
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| 8. | Later uses, along with " equidistant " and " directrix ", are vague and metaphorical.
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| 9. | This implies that, seen from any point on its directrix, any parabola subtends a right angle.
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| 10. | In this diagram, F is the focus of the parabola, and T and U lie on its directrix.
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