Question:

A particle is moving in the xy-plane along a curve C passing through the point (3, 3). The tangent to the curve C at the point P meets the x-axis at Q. If the y-axis bisects the segment PQ, then C is a parabola with

Updated On: Apr 7, 2024
  • Length of latus rectum 3
  • Length of latus rectum 6
  • \(Focus \bigg(\frac{4}{3},0\bigg)\)
  • \(Focus \bigg(0,\frac{3}{4}\bigg)\)
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The Correct Option is A

Solution and Explanation

\(2x-y \frac{dx}{dy} = 0\)

tangent at \(P:y-y =\frac{dy}{dx}(y-x)\)

\(∴ 2 \frac{dy}{y} = \frac{dx}{x}\)

\(⇒ 2Iny = Inx+Inc\)

\(⇒ y^2 = cx\)

 At coordinates \((3, 3)\) curves pass through
Hence, \(c = 3\)

Therefore, the is parabola : 

\(y^2 = 3x \)

So Length of latus rectum is \(3\).

Hence, the correct option is (A): Length of latus rectum is \(3\)

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Concepts Used:

Parabola

Parabola is defined as the locus of points equidistant from a fixed point (called focus) and a fixed-line (called directrix).

Parabola


 

 

 

 

 

 

 

 

 

Standard Equation of a Parabola

For horizontal parabola

  • Let us consider
  • Origin (0,0) as the parabola's vertex A,
  1. Two equidistant points S(a,0) as focus, and Z(- a,0) as a directrix point,
  2. P(x,y) as the moving point.
  • Let us now draw SZ perpendicular from S to the directrix. Then, SZ will be the axis of the parabola.
  • The centre point of SZ i.e. A will now lie on the locus of P, i.e. AS = AZ.
  • The x-axis will be along the line AS, and the y-axis will be along the perpendicular to AS at A, as in the figure.
  • By definition PM = PS

=> MP2 = PS2 

  • So, (a + x)2 = (x - a)2 + y2.
  • Hence, we can get the equation of horizontal parabola as y2 = 4ax.

For vertical parabola

  • Let us consider
  • Origin (0,0) as the parabola's vertex A
  1. Two equidistant points, S(0,b) as focus and Z(0, -b) as a directrix point
  2. P(x,y) as any moving point
  • Let us now draw a perpendicular SZ from S to the directrix.
  • Then SZ will be the axis of the parabola. Now, the midpoint of SZ i.e. A, will lie on P’s locus i.e. AS=AZ.
  • The y-axis will be along the line AS, and the x-axis will be perpendicular to AS at A, as shown in the figure.
  • By definition PM = PS

=> MP2 = PS2

So, (b + y)2 = (y - b)2 + x2

  • As a result, the vertical parabola equation is x2= 4by.