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JEE Advanced 2019 · Application of Derivatives

Define the collections {E1,E2,E3,…}\{E_1, E_2, E_3, \ldots\} of ellipses and {R1,R2,R3,…}\{R_1, R_2, R_3, \ldots\} of rectangles as follows : E1:x29+y24=1E_1 : \frac{x^2}{9} + \frac{y^2}{4} = 1 ; R1R_1 : rectangle of largest area, with sides parallel to the axes, inscribed in E1E_1; EnE_n : ellipse x2an2+y2bn2=1\frac{x^2}{a_n^2} + \frac{y^2}{b_n^2} = 1 of largest area inscribed in Rn−1R_{n-1}, n>1n > 1; RnR_n : rectangle of largest area, with sides parallel to the axes, inscribed in En,n>1E_n, n > 1. Then which of the following options is/are correct?

Answers: B. The length of latus rectum of E9E_9 is 16\frac{1}{6} · C. ∑n=1N\sum_{n=1}^{N} (area of RnR_n) <24< 24, for each positive integer NN

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Define the collections {E₁, E₂, E₃, …} of ellipses and {R₁, R₂, R₃, …} of rectangles as follows : E₁ : x²/9 + y²/4 = 1 ; R₁ : rectangle of largest area, with sides parallel to the axes, inscribed in E₁; Eₙ : ellipse x²/aₙ² + y²/bₙ² = 1 of largest area inscribed in Rₙ₋₁, n > 1; Rₙ : rectangle of largest area, with sides parallel to the axes, inscribed in Eₙ, n > 1. Then which of the following options is/are correct? E1:x29+y24=1E_1: \dfrac{x^2}{9} + \dfrac{y^2}{4} = 1 R1=largest rectangle in E1;En in Rn−1, Rn in EnR_1 = \text{largest rectangle in } E_1;\quad E_n \text{ in } R_{n-1},\ R_n \text{ in } E_n Which of (1) - (4) is/are correct?\text{Which of (1) - (4) is/are correct?} E1:a1=3,b1=2E_1: a_1 = 3,\quad b_1 = 2 max rectangle in x2a2+y2b2=1: area 2ab, corners (±a2,±b2)\text{max rectangle in } \dfrac{x^2}{a^2}+\dfrac{y^2}{b^2}=1:\ \text{area } 2ab,\ \text{corners } \left(\pm\dfrac{a}{\sqrt2},\pm\dfrac{b}{\sqrt2}\right) max ellipse in W×H rectangle:a=W2,b=H2\text{max ellipse in } W\times H \text{ rectangle}:\quad a = \dfrac{W}{2},\quad b = \dfrac{H}{2} W1=2⋅a12=32,H1=2⋅b12=22W_1 = 2\cdot\dfrac{a_1}{\sqrt2} = 3\sqrt2,\quad H_1 = 2\cdot\dfrac{b_1}{\sqrt2} = 2\sqrt2 area R1=2a1b1=12\text{area } R_1 = 2a_1b_1 = 12 Wn+1=Wn2,Hn+1=Hn2W_{n+1} = \dfrac{W_n}{\sqrt2},\qquad H_{n+1} = \dfrac{H_n}{\sqrt2} Wn=3⋅22−n2,Hn=2⋅22−n2W_n = 3\cdot 2^{\frac{2-n}{2}},\qquad H_n = 2\cdot 2^{\frac{2-n}{2}} bnan=Hn−1Wn−1=23 ∀n⇒e=1−49=53\dfrac{b_n}{a_n} = \dfrac{H_{n-1}}{W_{n-1}} = \dfrac{2}{3}\ \forall n \Rightarrow e = \sqrt{1-\dfrac49} = \dfrac{\sqrt5}{3} ⇒(1) FALSE\Rightarrow (1) \text{ FALSE} a9=316,b9=18a_9 = \dfrac{3}{16},\quad b_9 = \dfrac{1}{8} latus rectum=2b92a9=2/643/16=16⇒(2) TRUE\text{latus rectum} = \dfrac{2b_9^2}{a_9} = \dfrac{2/64}{3/16} = \dfrac16 \Rightarrow (2) \text{ TRUE} area Rn=WnHn=6⋅22−n\text{area } R_n = W_n H_n = 6\cdot 2^{2-n} ∑n=1∞6⋅22−n=6⋅4=24⇒each partial sum<24⇒(3) TRUE\sum_{n=1}^{\infty} 6\cdot 2^{2-n} = 6\cdot 4 = 24 \Rightarrow \text{each partial sum} < 24 \Rightarrow (3) \text{ TRUE} c=a92−b92=9256−164=516≠532c = \sqrt{a_9^2 - b_9^2} = \sqrt{\dfrac{9}{256} - \dfrac{1}{64}} = \dfrac{\sqrt5}{16} \ne \dfrac{\sqrt5}{32} ⇒(4) FALSE\Rightarrow (4) \text{ FALSE} (2) and (3) are correct\boxed{(2)\ \text{and}\ (3)\ \text{are correct}}

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