## Jonask Group Title prove that the directrix of an ellipse is $x=\frac{a^2}{c}$ one year ago one year ago

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

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

$r=\frac{ a-c }{ d-a }=\frac{ a }{ d }$

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

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??|dw:1364055606757:dw||dw:1364055629069:dw|

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

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what's r there?

its defined in the 1st link

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can't find it ... equation no?

24

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what is this 'c' again?

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this 'c' looks like foci.

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