For a positive real number k let Ek be the ellipse with equation x2a2+k+y2b2+k=1 where a>b>0. All members of the family of ellipse {Ek:k>0} have the same.
A
Foci
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B
Eccentricity
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C
Pair of directrices
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D
Centre
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Solution
The correct options are B Foci C Centre Given ellipse is x2a2+k+x2b2+k=1,
Eccentricity of the given ellipse is e2=b2−a2k+a2
This depends on k,
the focci are ±(ae,0)=±(√b2−a2,0),independent of k,∴ all the ellipse of this family have the same focci,