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JEE Main 2021
Conic Sections
Parabola
Medium

Question

A line is a common tangent to the circle (x - 3) 2 + y 2 = 9 and the parabola y 2 = 4x. If the two points of contact (a, b) and (c, d) are distinct and lie in the first quadrant, then 2(a + c) is equal to _________.

Answer: 3

Solution

This problem involves finding a common tangent line to a parabola and a circle, identifying their respective points of contact in the first quadrant, and then performing a specific calculation based on these coordinates. We will leverage the standard forms of tangent equations and the condition for tangency.


1. Key Concepts and Formulas

  • Equation of a Tangent to a Parabola: For a parabola of the standard form y2=4Axy^2 = 4Ax, the equation of a tangent line with slope mm is given by: y=mx+Amy = mx + \frac{A}{m} The coordinates of the point of contact (xP,yP)(x_P, y_P) for this tangent on the parabola are:

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