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JEE Advanced 2019Paper 1
Exam
JEE Advanced
Year
2019
Paper
Paper 1
Subject
Mathematics
Chapter
Ellipse

JEE Advanced 2019 · Paper 1 · Mathematics · Ellipse

Define the collections \ E _ 1 , E _ 2 , E _ 3 , . . . .\ of...

Define the collections {E1,E2,E3,....}\left\{{E}_{1}, {E}_{2}, {E}_{3}, . . . .\right\} of ellipses and \left\{{R}_{1}, {R}_{2}, {R}_{3}, . . . .\right\}ofrectanglesasfollows:<br/>of rectangles as follows:<br />{E}_{1}: \dfrac{{x}^{2}}{9} + \dfrac{{y}^{2}}{4} = 1;<br/><br />{R}_{1}:rectangleoflargestarea,withsidesparalleltotheaxes,inscribedinrectangle of largest area, with sides parallel to the axes, inscribed in{E}_{1};<br/><br />{E}_{n}:ellipseellipse\dfrac{{x}^{2}}{{a}_{n}^{2}} + \dfrac{{y}^{2}}{{b}_{n}^{2}} = 1oflargestareainscribedinof largest area inscribed in{R}_{n - 1}, n > 1;<br/><br />{R}_{n}:rectangleoflargestarea,withsidesparalleltotheaxes,inscribedinrectangle of largest area, with sides parallel to the axes, inscribed in{E}_{n}, n > 1 .
Then which of the following options is/are correct?Define the collections \ E _ 1 , E _ 2 , E _ 3 , . . . .\ of ellipses and \ R _ 1 , R _ 2 , R _ 3 , . . . .\ of

Options

  1. AThe eccentricities of E18{E}_{18} and E19{E}_{19} are NOT equal
  2. BThe distance of a focus from the centre in E9{E}_{9} is 532\dfrac{\sqrt{5}}{32}
  3. CThe length of latus rectum of E9{E}_{9} is 16\dfrac{1}{6}
  4. D∑n=1N(areaofRn)<24,\mathop{∑}_{n = 1}^{N}\left(areaof{R}_{n}\right) < 24, for each positive integer NN

Correct answer

C. The length of latus rectum of E9{E}_{9} is 16\dfrac{1}{6}

Step-by-step solution

As given:
E1=x2a2+y2b2=1{E}_{1} = \dfrac{{x}^{2}}{{a}^{2}} + \dfrac{{y}^{2}}{{b}^{2}} = 1 (Here a=3a = 3 and b=2b = 2 )
Semi major axis =a= a and semi minor axis =b= b
Let a vertex of R1{R}_{1} be \left(acos\theta, bsin\theta\right)<br>Thenareaof<br >Then area of{R}_{1} = 2acos\theta \times 2bsin\theta<br><br >{R}_{1} = 2absin2\theta<br>Maximumareaof<br >Maximum area of{R}_{1} = 2ab,</p><p>when</p><p>when\sin 2\theta = 1<br>or,<br >or,2\theta = \dfrac{\pi}{2}<br><br >\theta = \dfrac{\pi}{4}<br>Maximumareaof<br >Maximum area of{R}_{1} = 2ab....(i)<br><imgsrc="https://cdn−question−pool.getmarks.app/pyq/jeeadvanced/akloqbgqpbohzxzokfhiqtzcxhgxaiegqspfcliktkbppqtbvz.png"/><br>Now,ellipse....(i)<br ><img src="https://cdn-question-pool.Quantrex.app/pyq/jee_advanced/akloqbgqpbohzxzokfhiqtzcxhgxaiegqspfcliktkbppqtbvz.png"/><br >Now, ellipse{E}_{2}willhavesemimajoraxiswill have semi major axis\dfrac{a}{\sqrt{2}}andsemiminoraxisand semi minor axis\dfrac{b}{\sqrt{2}}<br><br >∵ {E}_{2} = \dfrac{{x}^{2}}{{\left(\dfrac{a}{\sqrt{2}}\right)}^{2}} + \dfrac{{y}^{2}}{{\left(\dfrac{b}{\sqrt{2}}\right)}^{2}} = 1,</p><p>hencemaximumareaof</p><p>hence maximum area of{R}_{2} = 2\left(\dfrac{a}{\sqrt{2}}\right)(b2)\left(\dfrac{b}{\sqrt{2}}\right)
Similarly, ellipse E3{E}_{3} will have semi major axis a(2)2\dfrac{a}{{\left(\sqrt{2}\right)}^{2}} and semi minor axis b(2)2\dfrac{b}{{\left(\sqrt{2}\right)}^{2}}
E3:x2(a(2)2)2+y2(b(2))2=1{E}_{3}: \dfrac{{x}^{2}}{{\left(\dfrac{a}{{\left(\sqrt{2}\right)}^{2}}\right)}^{2}} + \dfrac{{y}^{2}}{{\left(\dfrac{b}{\left(\sqrt{2}\right)}\right)}^{2}} = 1
And maximum area of
Rn=2(a(2)n−1)(b(2)n−1){R}_{n} = 2\left(\dfrac{a}{{\left(\sqrt{2}\right)}^{n - 1}}\right)\left(\dfrac{b}{{\left(\sqrt{2}\right)}^{n - 1}}\right)
Option (1):( 1 ) :
All ellipse will have some eccentricity as the ratios of semi major axis and semi minor axis is same for all ellipse.
e=1−bn2an2=1−b2a2=53e = \sqrt{1 - \dfrac{{b}_{n}^{2}}{{a}_{n}^{2}}} = \sqrt{1 - \dfrac{{b}^{2}}{{a}^{2}}} = \dfrac{\sqrt{5}}{3}

Option (4):( 4 ) :
∑n=1marea of rectangleRn=R1+R2+…..+Rm\mathop{∑}_{n = 1}^{m}\text{area of rectangle}{R}_{n} = {R}_{1} + {R}_{2} + \ldots . . + {R}_{m}
Area of mm rectangle will be lesser than area of infinite
Rectangle ⇒R1+R2+R2+….+Rm<R1+R2+R3+…..∞⇒ {R}_{1} + {R}_{2} + {R}_{2} + \ldots . + {R}_{m} < {R}_{1} + {R}_{2} + {R}_{3} + \ldots . . \infty
∑n=1marea of rectangle<2ab+2(a2)(b2)+.....∞=2ab[1+1(2)2+.....∞]\mathop{∑}_{n = 1}^{m}\text{area of rectangle} < 2ab + 2\left(\dfrac{a}{\sqrt{2}}\right)\left(\dfrac{b}{\sqrt{2}}\right) + . . . . . \infty = 2ab\left[1 + \dfrac{1}{{\left(\sqrt{2}\right)}^{2}} + . . . . . \infty\right]
∑n=1mareaofrectangeRn<2ab[11-12]\mathop{∑}_{n = 1}^{m}areaofrectange{R}_{n} < 2ab\left[\dfrac{\text{1}}{\text{1-}\dfrac{\text{1}}{\text{2}}}\right]
∑n=1mareaofrectangeRn<4ab\mathop{∑}_{n = 1}^{m}areaofrectange{R}_{n} < 4ab
∑n=1mareaofrectangeRn<4×3×2\mathop{∑}_{n = 1}^{m}areaofrectange{R}_{n} < 4 \times 3 \times 2
∑n=1mareaofrectangeRn<24\mathop{∑}_{n = 1}^{m}areaofrectange{R}_{n} < 24
Option (3):( 3 ) :
Length of latus rectum of En=2bn2an2=2b2a(2)n−1{E}_{n} = \dfrac{2{b}_{n}^{2}}{{a}_{n}^{2}} = \dfrac{2{b}^{2}}{a{\left(\sqrt{2}\right)}^{n - 1}}
L.R.ofEg=2b2a(2)2=2(2)23×24=16L . R . of{E}_{g} = \dfrac{2{b}^{2}}{a{\left(\sqrt{2}\right)}^{2}} = \dfrac{2{\left(2\right)}^{2}}{3 \times {2}^{4}} = \dfrac{1}{6}
Option (2):( 2 ) :
Distance between focus and centre of E9=a9e{E}_{9} = {a}_{9}e
=a(2)2×53= \dfrac{a}{{( \sqrt{2} )}^{2}} \times \dfrac{\sqrt{5}}{3}
=316×53= \dfrac{3}{16} \times \dfrac{\sqrt{5}}{3}
=516= \dfrac{\sqrt{5}}{16}

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