Tue September 20, 2011 By:

dear sir, on the ellipse again, why is the directrix taken as x = a/e

Expert Reply
Tue September 27, 2011
An ellipse is locus or path of a point which moves in such a way that its dis tance from a fixed point called focus (denoted by a)bears a constant ratio to its distance from from a fixed line called a directrix (denoted by a).The ratio of these distances denoted by e, is called the eccentricity of the ellipse and this ratio e is always less than one.
=> e=a/x
Therefore, x=a/e
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