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10-2. Parabola, Ellipse, Hyperbola
normal
For hyperbola $\frac{{{x^2}}}{{{{\cos }^2}\alpha }} - \frac{{{y^2}}}{{{{\sin }^2}\alpha }} = 1$ which of the following remain constant if $\alpha$ varies
A
Eccentricity
B
Directrix
C
Abscissae of vertices
D
Abscissae of foci
Solution
$\mathrm{a}^{2}=\cos ^{2} \alpha, \mathrm{b}^{2}=\sin ^{2} \alpha$
$e=\sqrt{1+\frac{b^{2}}{a^{2}}}=\frac{1}{\cos \alpha}$
Abscissa of foci $\equiv(a e, 0)=(1,0)$ which is independent of $\alpha .$
Standard 11
Mathematics