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JEE Main 2023
Conic Sections
Parabola
Hard

Question

Let A,BA, B and CC be three points on the parabola y2=6xy^2=6 x and let the line segment ABA B meet the line LL through CC parallel to the xx-axis at the point DD. Let MM and NN respectively be the feet of the perpendiculars from AA and BB on LL. Then (AMBNCD)2\left(\frac{A M \cdot B N}{C D}\right)^2 is equal to __________.

Answer: 1

Solution

This problem is a classic example of how parametric representation simplifies calculations in coordinate geometry, especially for conic sections like parabolas. The key is to express all points and derived geometric properties in terms of the parameters, which often leads to elegant cancellations.


1. Introduction: Parametric Representation of a Parabola

The most efficient way to approach problems involving points on a parabola, particularly when dealing with chords, tangents, and geometric relationships, is by using parametric coordinates. This method reduces the number of variables and often reveals underlying symmetries and simplifications.

The given parabola is y2=6xy^2 = 6x. The standard form of a parabola with its vertex at the origin and axis along the x-axis is y2=4axy^2 = 4ax. By comparing $y

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