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Find the equation of the ellipse if the length of the semi major axis, and the latus rectum are $7$ and $\large\frac{80}{7}$ respectively, the centre is $(2 , 5 )$ and the major axis is parallel to y- axis.

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Toolbox:
  • Standard forms of equation of the ellipse with major axes 2a, minor axis 2b $(a >b)$ eccentricity e and $b^2=a^2(1-e^2)$ or $e^2=1-\large\frac{b^2}{a^2}$
  • $\large\frac{x^2}{a^2}+\frac{y^2}{b^2}$$=1$
  • http://clay6.com/mpaimg/1_2_Toolbox.png
  • Foci$(\pm ae,o),$ center $(0,0)$,vertices $(\pm a,0)$
  • End points of Latus Rectum $\; (ae,\pm \large\frac{b^2}{a})$ and $(-ae,\pm \large\frac{b^2}{a})$
  • Directrices $x=\pm \large\frac{a}{e}$.
  • The major axis is $y=0$ (x- axis) and the minor axes is $x=0$ (y- axis)
  • $\large\frac{x^2}{b^2}+\frac{y^2}{a^2}$$=1$
  • http://clay6.com/mpaimg/1_2_Toolbox1.png
  • Foci$(0,\pm ae),$ center $(0,0)$,vertices $(0,\pm a)$
  • End points of Latus Rectum $(\pm \large\frac{b^2}{a}$$,ae)$ and $(\pm \large\frac{b^2}{a}$$,-ae)$
  • Directrices $y=\pm \large\frac{a}{e}$.
  • The major axis is $x=0$ (y- axis) and the minor axes is $y=0$ (x- axis)
  • General form of standard ellipses with centre $C$ with major axis $2a$,minor axis $2b$ and axes parallel to the coordinate axes.
  • $\large\frac{(x-h)^2}{a^2}+\frac{(y-k)^2}{b^2}$$=1$
  • In this case major axis y=k and minor axis $x=h$.
  • $\large\frac{(x-h)^2}{b^2}+\frac{(y-k)^2}{a^2}$$=1$
  • In this case major axis $x=h$ and minor axis $y=k$.
Step 1:
$a=7$,length of $LR=\large\frac{80}{7}=\frac{2b^2}{a}$
$\Rightarrow 2b^2=\large\frac{80}{7}=$$80$
$b^2=40$
$b=2\sqrt {10}$
step 2:
The centre is $ab(2,5)$ and the major axis is parallel to $y$-axis.
The equation is $\large\frac{(x-2)^2}{b^2}+\frac{(y-5)^2}{a^2}$$=1$
$\Rightarrow \large\frac{(x-2)^2}{40}+\frac{(y-5)^2}{49}$$=1$
This is the required equation.
answered Jun 17, 2013 by sreemathi.v
 

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