Jonask
prove that the directrix of an ellipse is \[x=\frac{a^2}{c}\]



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Jonask
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this ellipse is the general ellipse with \[\frac{ x^2 }{ a^2}+\frac{ y^2 }{ b^2 }=1,a>b\]

Jonask
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i have the proof but i dont understand the ratio set up here can i post the link


Jonask
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\[r=\frac{ ac }{ da }=\frac{ a }{ d }\]

Jonask
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\[a^2=b^2+c^2
i dont get this ratio .why it holds

experimentX
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??dw:1364055606757:dwdw:1364055629069:dw

experimentX
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i haven't done conics for quite while, but this shouldn't be difficult.dw:1364055735803:dw


experimentX
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what's r there?

Jonask
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its defined in the 1st link

experimentX
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can't find it ... equation no?

Jonask
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24

experimentX
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what is this 'c' again?

experimentX
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this 'c' looks like foci.

experimentX
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this is pretty ovbioius ... this ratio must be constant