Let S and S’ be the foci of the ellipse and
Question:

Let $S$ and $S^{\prime}$ be the foci of the ellipse and $B$ be any one of the extremities of its minor axis. If $\Delta \mathrm{S} ‘ \mathrm{BS}$ is a right angled triangle with right angle at $B$ and area $\left(\Delta S^{\prime} B S\right)=8$ sq. units, then the length of a latus rectum of the ellipse is :

  1. $2 \sqrt{2}$

  2. 2

  3. 4

  4. $4 \sqrt{2}$


Correct Option: , 3

Solution:

$\mathrm{m}_{\mathrm{SB}} \cdot \mathrm{m}_{\mathrm{SB}}=-1$

$b^{2}=a^{2} e^{2} \quad \ldots .$ (i)

$\frac{1}{2} S^{\prime} B \cdot S B=8$

S’B. $\mathrm{SB}=16$

$a^{2} e^{2}+b^{2}=16 \ldots . .$ (ii)

$b^{2}=a^{2}\left(1-e^{2}\right) \ldots .$ (iii)

using (i),(ii), (iii) $a=4$

$b=2 \sqrt{2}$

$e=\frac{1}{\sqrt{2}}$

$\therefore \ell($ L.R $)=\frac{2 b^{2}}{\mathrm{a}}=4 \quad$ Ans.3

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