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Q.

Statement – 1:Tangents PA and PB are drawn to parabola, where A and B are points of contacts of tangents. If mid-point of chord AB is M then line PM is perpendicular to tangent at vertex.
Statement 2:  Maximum distance of a normal drawn to ellipse x2a2+y2b2=1(a>b>0) at any point from its centre is a + b.
 Statement 3:  The normal to the ellipse x2a2+y2b2=1 at a point ‘θ’ is axcosθbysinθ=a2b2

Statement 4: The radius of the largest circle having center at focus of ellipse x2a2+y2b2=1(a>b>0) and inscribed in it is a(1 – e) where e is eccentricity of ellipse.
Which of the following must be truth value of above statements in that order.

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a

FTTF

b

TTTF

c

TFTT

d

TFTF

answer is A.

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Detailed Solution

For hte statement 1

The tangents at the points At1,Bt2

The point of intersection of tangents is Pat1t2,at1+t2 and midpoint of AB is Mat12+t222,at2+t2
The tangent to the parabola at the vertex is y- axis, and the line PM is parallel to x- axis,

Hence, PM is perpendicular to the tangent at the vertex. 


-For the statement 2

The equation of normal to the ellipse is axcosϑ-bysinϑ=a2-b2

The perpendicular distance from origin to the above line is a2-b2a2sec2ϑ+b2cosec2ϑ

To get the maximum value of the above value, let us consider the minimum value of denomiantor, it is a+b

Hence, the required value is a-b


For the statement 3, It is formula


For the statement 4

The equatio of the circle is x-ae2+y2=r2 and this touches the ellipse x2a2+y2b2=1-

Substitute y2=b21-x2a2

x2+a2e2-2aex+b2-b2a2x2-r2=0

It is a quadratic equation, it has only one root

4a2e2=41-b2a2a2e2-r2 =4e2a2e2-r2a2=a2e2-r2r=ae2-1

Hence, statement 1, Statement 2, Statement 3 are correct, but statement 4 is wrong. 

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