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Find the equation of the ellipse referred to its centre whose minor axis is equal to the distance between the foci and whose latus is 10?

$\begin{array}{1 1}(a)\;\large\frac{x^2}{100}+\frac{y^2}{50}\normalsize=1\\(b)\;\large\frac{x^2}{10}+\frac{y^2}{5}\normalsize=1\\(c)\;\large\frac{x^2}{200}+\frac{y^2}{100}\normalsize=1\\(d)\;\text{None of these}\end{array}$

1 Answer

Let the equation of the ellipse is $\large\frac{x^2}{a^2}+\frac{y^2}{b^2}$$=1\qquad a > b$
Then the foci are $S(ae,0)$ and $S'(-ae,0)$
Length of minor axes =2b
Length of lactus rectum =$\large\frac{2b^2}{a}$
According to the question
$e=\large\frac{1}{\sqrt 2}$
$b=\large\frac{a}{\sqrt 2}$
$b^2=5\times 10=50$
Hence equation of ellipse
Hence (a) is the correct answer.
answered Feb 7, 2014 by sreemathi.v