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Latus Rectum - Practice Questions & MCQ

Edited By admin | Updated on Sep 18, 2023 18:34 AM | #JEE Main

Quick Facts

  • Length of Latusrectum is considered one of the most asked concept.

  • 46 Questions around this concept.

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Consider an ellipse, whose centre is at the origin and its major axis is along the $x$-axis. If its eccentricity is $\frac{3}{5}$ and the distance between its foci is 6 , then the area (in sq. units) of the quadrilateral inscribed in the ellipse, with the vertices as the end points of major and minor axes of ellipse, is

If O is the origin and OB is the semi-minor axis of an ellipse, F1 and F2 are its foci and the angle between F1B and F2B is a right angle, then the square of the eccentricity of the ellipse is

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

If the latus rectum of the ellipse $\mathrm{x}^2 \tan ^2 \alpha+\mathrm{y}^2 \sec ^2 \alpha=1$ is $\frac{1}{2}$, then $\alpha\left(0<\alpha<\frac{\pi}{3}\right)$ is equal to:

In an ellipse the distance between its foci is 6 and its minor axis is 8. Then its eccentricity is 

The distance of the point \mathrm{' \theta^{\prime}}  on the ellipse \mathrm{\frac{x^2}{a^2}+\frac{y^2}{b^2}=1} from a focus is

$
\text { What is the length of the latus rectum of the ellipse: } 9 x^2+25 y^2=225
$

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 The length of the latus rectum of the ellipse 3x+ y2 = 12 is
 

If the latus rectum of an ellipse is equal to half the minor axis then its eccentricity is equal to

 

 

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Concepts Covered - 1

Length of Latusrectum

Length of Latus rectum

Let Latus rectum $\mathrm{LL}^{\prime}=2 \alpha$
$\mathrm{S}(\mathrm{ae}, 0)$ is focus, then $\mathrm{LS}=\mathrm{SL}^{\prime}=\alpha$
Coordinates of L and $\mathrm{L}^{\prime}$ become ( $\mathrm{ae}, \alpha$ ) and (ae, $-\alpha$ ) respectively
Equation of ellipse, $\frac{x^2}{a^2}+\frac{y^2}{b^2}=1$
Put $x=a e, y=\alpha$ we get, $\frac{(\mathrm{ae})^2}{\mathrm{a}^2}+\frac{\alpha^2}{\mathrm{~b}^2}=1 \Rightarrow \alpha^2=\mathrm{b}^2\left(1-\mathrm{e}^2\right)$

$
\begin{aligned}
& \alpha^2=\mathrm{b}^2\left(\frac{\mathrm{~b}^2}{\mathrm{a}^2}\right) \quad\left[\mathrm{b}^2=\mathrm{a}^2\left(1-\mathrm{e}^2\right)\right] \\
& \alpha=\frac{\mathrm{b}^2}{\mathrm{a}} \\
& \Rightarrow 2 \alpha=\mathrm{LL}^{\prime}=\frac{2 \mathrm{~b}^2}{\mathrm{a}}
\end{aligned}
$

End Points of Latus rectum

$\mathrm{L}=\left(\mathrm{ae}, \frac{\mathrm{v}}{\mathrm{a}}\right)$ and $\mathrm{L}^{\prime}=\left(\mathrm{ae},-\frac{\mathrm{v}}{\mathrm{a}}\right)$

Focal Distance of a Point:

The sum of the focal distance of any point on the ellipse is equal to the major axis.

Let $\mathrm{P}(\mathrm{x}, \mathrm{y})$ be any point on the ellipse.

Here, $\quad \begin{aligned} & S P=e P M=e\left(\frac{a}{e}-x\right)=a-e x \\ & \\ & S^{\prime} P=e P M^{\prime}=e\left(\frac{a}{e}+x\right)=a+e x\end{aligned}$

Now, $\mathrm{SP}+\mathrm{S}^{\prime} \mathrm{P}=\mathrm{a}-\mathrm{ex}+\mathrm{a}+\mathrm{ex}=2 \mathrm{a}=\mathrm{AA}^{\prime}=$ constant.
Thus the sum of the focal distances of a point on the ellipse is constant.

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Length of Latusrectum

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