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Let "a" and "b" be the major and minor semi-axes of the ellipse, respectively.
Then from the condition you have these two equations
(1) (9+a) + (a-9) = 30 ( <<<---=== "the sum of the focal radii = 30" )
( it is the sum of the distances from foci to the vertex of the ellipse !)
which becomes 2a = 30 ====> a = 15.
(2) = 9 ( <<<---=== the distance from the center of the ellipse to its focus is 9).
Then (2) implies = = = 144,
hence, b = = 12.
Thus the semi-axes are 15 and 12 units long, and the equation of the ellipse is
+ = 1.
Solved.
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See the lesson
- Ellipse definition, canonical equation, characteristic points and elements
in this site.