The Interactive Geometry Software Cinderella
Forums-> Cinderella Support (E)-> Unable to construct a parabola by focus and directrix-> Re: Unable to construct a parabola by focus and directrix
DavidBakin
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Re: Unable to construct a parabola by focus and directrix
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Definitely a bug, but not in construction - perhaps in drawing.
If you look at the construction window you can see your parabola defined - twice! It is both C3 and C6. And as you move point g, thereby moving point F, you can see the equations for C3 and C6 change. But the parabola isn't drawn.
Neither is the parabola C2 - with G as focus as k as directrix - except for a certain range of point g - as you move g you can see C2 appear and disappear.
Finally, if you just define an arbitrary free point, and then define a parabola using it as focus and k as directrix, you can also see the parabola appear and disappear as you move the free point - sometimes. (For me it seems to depend on the position of point g - sometimes the parabola off the free point never disappears, but sometimes, e.g., when C2 is visible, it does sometimes disappear.
Very curious! — David
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