Q.

The normal at a variable point P on ellipse x2a2+y2b2=1, (a > b) meets axes of the ellipse in Q and R. Then which of the following is/are correct statement(s)

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a

 the locus of mid-point of QR is conic whose eccentricity is same as of given ellipse

b

 The locus of mid-point of QR is hyperbola

c

the locus of mid-point of QR is ellipse

d

 The locus of mid-point of QR is conic with its eccentricity half of the eccentricity of given ellipse

answer is A, B.

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Detailed Solution

 Equation of Normal to the Ellipse x2a2+y2b2=1 at P(acosθ,bsinθ) is a2xacosθb2ybsinθ=a2-b2axcosθbysinθ=a2-b2 it cuts x -aris at QQ=a2-b2cosθa,0 and Normal cuts y -axis at RR0,-a2-b2sinθb Let x1,y1 be the mid point of QR Hence x1=a2-b2cosθ2a,y1:-a2-b2sinθ2bcosθ=2ax1a2-b2sinθ=-2by1a2-b2 Eliminate θ ,4a2x12a2-b22+4b2y12a2-b22=1x12a2-b224a2+y12a2-b224b2=1  The locus of mid point is x2a4e44a2+y2a4e44b2=1 This represents Ellipse Eccentricity e=1-1a21b2=1-b2a2  Same as that of given Ellipse 

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