Engineering
Mathematics
Standard Ellipse
Question

Define the collections {E1, E2, E3, ….…} of ellipses and {R1, R2, R3, ….…} of rectangles as follows:

E1: x29+y24 = 1  ;

R1 : rectangle of largest area, with sides parallel to the axes, inscribed in E1 ;

En : ellipse x2an2+y2bn2 = 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?

The distance of a focus from the centre in E9 is 532.

∑n=1N(area  of  Rn) < 24, for each positive integer N

The length of latus rectum of E9 is 16.

The eccentricities of E18 and E19 are NOT equal.

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Solution

Area of R1 = 3sin 2θ, for this to be maximum

⇒θ=π4⇒(32, 22)                                      

Hence for subsequent areas of rectangles Rn to be maximum the coordinates will be in G.P. with common ratio r=12⇒  an=3( 2 ) n−1;  bn=3( 2 ) n−1

Eccentricity of all the ellipses will be same

Distance of a focus from the centre in E9 = a9e9 = a92−b92=516

Length of latus rectum of E9 = 2b92a9=16

∑n=1∞Area of Rn=12+122+124+.........∞=24

∑n=1N(area of Rn) < 24, for each positive integer N