1.

For the hyperbola `(x^2)/(cos^2alpha)-(y^2)/(sin^2alpha)=1`, which ofthe following remains constant when `alpha`varies?(1)eccentricity(2) directrix(3) abscissae of vertices (4) abscissaeof fociA. Abscissae of verticesB. Abscissae of fociC. EccentricityD. Directrix

Answer» Given equation of hyperbola is `(x^(2))/(cos^(2)alpha)-(y^(2))/(sin^(2)alpha)=1 `
Here, `a^(2)=cos^(2)alpha and b^(2)=sin^(2)alpha`
[i.e. comparing with standard equation `(x^(2))/(a^(2))-(y^(2))/(b^(2))=1`]
We know that, foci `=(pm ae, 0)`
where, `ae=sqrt(a^(2)+b^(2))=sqrt(cos^(2)alpha+sin^(2)alpha)=1`
`rArr "Foci"=(pm 1, 0)`
where, vertices are `(pm cos alpha,0).`
Eccentricity, `ae=1 or e=(1)/(cos alpha)`
Hence, foci remains constant with change in `alpha`.


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