JEE Advanced 2019 · Paper 1 · Mathematics · Ellipse
Define the collections \ E _ 1 , E _ 2 , E _ 3 , . . . .\ of...
Define the collections {E1,E2,E3,....} of ellipses and \left\{{R}_{1}, {R}_{2}, {R}_{3}, . . . .\right\}ofrectanglesasfollows:<br/>{E}_{1}: \dfrac{{x}^{2}}{9} + \dfrac{{y}^{2}}{4} = 1;<br/>{R}_{1}:rectangleoflargestarea,withsidesparalleltotheaxes,inscribedin{E}_{1};<br/>{E}_{n}:ellipse\dfrac{{x}^{2}}{{a}_{n}^{2}} + \dfrac{{y}^{2}}{{b}_{n}^{2}} = 1oflargestareainscribedin{R}_{n - 1}, n > 1;<br/>{R}_{n}:rectangleoflargestarea,withsidesparalleltotheaxes,inscribedin{E}_{n}, n > 1 . Then which of the following options is/are correct?
Options
AThe eccentricities of E18 and E19 are NOT equal
BThe distance of a focus from the centre in E9 is 325
CThe length of latus rectum of E9 is 61
D∑n=1N(areaofRn)<24, for each positive integer N
Correct answer
C. The length of latus rectum of E9 is 61
Step-by-step solution
As given: E1=a2x2+b2y2=1 (Here a=3 and b=2 ) Semi major axis =a and semi minor axis =b Let a vertex of R1 be \left(acos\theta, bsin\theta\right)<br>Thenareaof{R}_{1} = 2acos\theta \times 2bsin\theta<br>{R}_{1} = 2absin2\theta<br>Maximumareaof{R}_{1} = 2ab,</p><p>when\sin 2\theta = 1<br>or,2\theta = \dfrac{\pi}{2}<br>\theta = \dfrac{\pi}{4}<br>Maximumareaof{R}_{1} = 2ab....(i)<br><imgsrc="https://cdn−question−pool.getmarks.app/pyq/jeeadvanced/akloqbgqpbohzxzokfhiqtzcxhgxaiegqspfcliktkbppqtbvz.png"/><br>Now,ellipse{E}_{2}willhavesemimajoraxis\dfrac{a}{\sqrt{2}}andsemiminoraxis\dfrac{b}{\sqrt{2}}<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{R}_{2} = 2\left(\dfrac{a}{\sqrt{2}}\right)(2b) Similarly, ellipse E3 will have semi major axis (2)2a and semi minor axis (2)2b E3:((2)2a)2x2+((2)b)2y2=1 And maximum area of Rn=2((2)n−1a)((2)n−1b) Option (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−an2bn2=1−a2b2=35
Option (4): ∑n=1marea of rectangleRn=R1+R2+…..+Rm Area of m rectangle will be lesser than area of infinite Rectangle ⇒R1+R2+R2+….+Rm<R1+R2+R3+…..∞ ∑n=1marea of rectangle<2ab+2(2a)(2b)+.....∞=2ab[1+(2)21+.....∞] ∑n=1mareaofrectangeRn<2ab1-211 ∑n=1mareaofrectangeRn<4ab ∑n=1mareaofrectangeRn<4×3×2 ∑n=1mareaofrectangeRn<24 Option (3): Length of latus rectum of En=an22bn2=a(2)n−12b2 L.R.ofEg=a(2)22b2=3×242(2)2=61 Option (2): Distance between focus and centre of E9=a9e =(2)2a×35 =163×35 =165