Q. The foci of an ellipse are $(0, \pm 4)$ and the equations for the directrices are $y = \pm 9$. The equation for the ellipse is

A) $5x^{2} + 9y^{2} = 4$
B) $2x^{2} - 6y^{2} = 28$
C) $6x^{2} + 3y^{2} = 45$
D) $9x^{2} + 5y^{2} = 180$
Explanation

(d)
Given foci of ellipse are $(0, - 4)$and (0, 4)
$\therefore$ Focal distance is$\ 2be = 8$
$\ \ be = 4\$ …(i)
Also, since equation of directrices are $\frac{b}{e} = \pm 9$ ...(ii)
From, Eqs. (i) and (ii), we get
$b^{2} = 36\ \Rightarrow \ b = 6\$and$\ e = \frac{2}{3}$
$\because\ a^{2} = b^{2}\left( 1 - e^{2} \right) = 36\ \left( 1 - \frac{4}{9} \right) = 20$
$\therefore\ \frac{x^{2}}{20} + \frac{y^{2}}{36} = 1$
$\Rightarrow \ 9x^{2} + 5y^{2} = 180$

Dr. Abubacker
Reviewed by Dr. Abubacker

Physics Educator & NEET/JEE Specialist

Ph.D. in Physics with 19+ years of teaching experience.

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