jee-advanced 2019 Q9
Infinite Series or Sequences Involving Circles
Define the collections $\left\{ E _ { 1 } , E _ { 2 } , E _ { 3 } , \ldots \right\}$ of ellipses and $\left\{ R _ { 1 } , R _ { 2 } , R _ { 3 } , \ldots \right\}$ of rectangles as follows: $E _ { 1 } : \frac { x ^ { 2 } } { 9 } + \frac { y ^ { 2 } } { 4 } = 1 ;$ $R _ { 1 }$ : rectangle of largest area, with sides parallel to the axes, inscribed in $E _ { 1 }$; $E _ { n }$ : ellipse $\frac { x ^ { 2 } } { a _ { n } ^ { 2 } } + \frac { y ^ { 2 } } { b _ { n } ^ { 2 } } = 1$ of largest area inscribed in $R _ { n - 1 } , n > 1$; $R _ { n }$ : rectangle of largest area, with sides parallel to the axes, inscribed in $E _ { n } , n > 1$. Then which of the following options is/are correct?
(A) The eccentricities of $E _ { 18 }$ and $E _ { 19 }$ are NOT equal
(B) $\quad \sum _ { n = 1 } ^ { N } \left( \right.$ area of $\left. R _ { n } \right) < 24$, for each positive integer $N$
(C) The length of latus rectum of $E _ { 9 }$ is $\frac { 1 } { 6 }$
(D) The distance of a focus from the centre in $E _ { 9 }$ is $\frac { \sqrt { 5 } } { 32 }$