Question

A focus of an ellipse is at the origin. The directrix is the line x = 4 and the eccentricity is 1/2. Then thelength of the semi−major axis is

Answer 1

@experimentX @hartnn @atlas @Psymon @zepdrix

Answer 2

Lol, never seen directrix used for an ellipse, so Im all dumb xD

Answer 3

@satellite73 @terenzreignz @.Sam. @experimentX

Answer 4

@atlas

Answer 5

let center by (x1, 0) and the ellipse cut x-axis at (x2,0).
your length of semi major axis is (x2 -x1)

you know that foci = ae = |(x2 - x1)| 1/2 = |x1|
and 1/2 |(4-x2)| = |x2 - x1|
solve this two equations and get |x2-x1|
this is your semi major axis.