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JEE Main 2024
Conic Sections
Parabola
Hard

Question

Let L1,L2L_1, L_2 be the lines passing through the point P(0,1)P(0,1) and touching the parabola 9x2+12x+18y14=09 x^2+12 x+18 y-14=0. Let QQ and RR be the points on the lines L1L_1 and L2L_2 such that the PQR\triangle P Q R is an isosceles triangle with base QRQ R. If the slopes of the lines QRQ R are m1m_1 and m2m_2, then 16(m12+m22)16\left(m_1^2+m_2^2\right) is equal to __________.

Answer: 9

Solution

This problem elegantly combines concepts of parabolas, tangents, and properties of isosceles triangles. We'll start by standardizing the parabola's equation, then find the equations of the tangent lines from the given point, and finally use the geometric properties of the isosceles triangle to determine the slopes of the base.


1. Standardizing the Parabola Equation

Concept: To work effectively with a parabola, we often transform its general equation into a standard form, typically (xh)2=4a(yk)(x-h)^2 = 4a(y-k) or (yk)2=4a(xh)(y-k)^2 = 4a(x-h). This helps identify the vertex, axis, and focal length, which are crucial for subsequent calculations.

Step-by-step working: The given equation of the

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