Question:

Let O be the vertex, Q be any point on the parabola x2=8y. If point P divides the line segment OQ internally in the ratio 1:3, then the locus of P is 

Updated On: Apr 11, 2025
  • x2=y
  • y2=x
  • y2=2x
  • x2=2y
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The Correct Option is D

Solution and Explanation

Given: Parabola with equation: \[ x^2 = 8y \] Any point on this parabola can be represented as: \[ Q = (4t, 2t^2) \]

Let O = (0, 0) and point P divides the line segment OQ in the ratio 1:3 internally.

Using section formula:
Coordinates of P (h, k) are: \[ h = \frac{1 \cdot 4t + 3 \cdot 0}{1 + 3} = \frac{4t}{4} = t \] \[ k = \frac{1 \cdot 2t^2 + 3 \cdot 0}{1 + 3} = \frac{2t^2}{4} = \frac{t^2}{2} \] So, P = (t, t^2/2)

Now eliminate parameter t: 
From \( h = t \), we get \( t = h \).
Substitute into k: \[ k = \frac{h^2}{2} \]

Hence, the locus of point P is: \[ y = \frac{x^2}{2} \Rightarrow x^2 = 2y \]

Final Answer: Option (D): \( x^2 = 2y \)

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Concepts Used:

Conic Sections

When a plane intersects a cone in multiple sections, several types of curves are obtained. These curves can be a circle, an ellipse, a parabola, and a hyperbola. When a plane cuts the cone other than the vertex then the following situations may occur:

Let ‘β’ is the angle made by the plane with the vertical axis of the cone

  1. When β = 90°, we say the section is a circle
  2. When α < β < 90°, then the section is an ellipse
  3. When α = β; then the section is said to as a parabola
  4. When 0 ≤ β < α; then the section is said to as a hyperbola

Read More: Conic Sections