Browse Questions

# 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.

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.