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JEE Main 2023
Conic Sections
Ellipse
Hard

Question

A line passing through the point P(5,5)P(\sqrt{5}, \sqrt{5}) intersects the ellipse x236+y225=1\frac{x^2}{36}+\frac{y^2}{25}=1 at AA and BB such that (PA)(PB)(P A) \cdot(P B) is maximum. Then 5(PA2+PB2)5\left(P A^2+P B^2\right) is equal to :

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Solution

This problem is a classic example of how the parametric form of a line can be a highly efficient and powerful tool for solving problems involving distances from a fixed point to the intersection points of a line and a conic section. This method allows us to directly obtain the distances as roots of a quadratic equation, simplifying calculations significantly, especially when dealing with products or sums of these distances.


1. Key Concept: Parametric Representation of a Line and its Intersection with a Conic

Consider a line passing through a fixed point P(x1,y1)P(x_1, y_1). If this line makes an angle θ\theta with the positive x-axis, any point (x,y)(x, y) on this line can be represented in its parametric form as: xx1cosθ=yy1sinθ=r\frac{x - x_1}{\cos \theta} = \frac{y - y_1}{\sin \theta} = r From this, we can express xx and yy coordinates in terms of rr: x=x1+rcosθx = x_1 + r \cos \theta y=y1+rsinθy = y_1 + r \sin \theta Here, rr represents the directed distance from the fixed point $P(

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