Question:medium

Let P be a point on the ellipse $\frac{x^2}{9}+\frac{y^2}{4}=1$ and let the perpendicular drawn through P to the major axis meet its auxiliary circle at Q. If the normals drawn at P and Q to the ellipse and the auxiliary circle respectively meet in R, then the equation of the locus of R is

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This problem has a standard result: the locus of the intersection of normals to an ellipse and its auxiliary circle at corresponding points P and Q is the circle $x^2+y^2=(a+b)^2$. Here, $a=3, b=2$, so the locus is $x^2+y^2=(3+2)^2=25$. Knowing this result can lead to the answer instantly.
Updated On: Mar 26, 2026
  • $x^2+y^2=5$
  • $x^2+y^2=13$
  • $x^2+y^2=25$
  • $x^2+y^2=1$
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The Correct Option is C

Solution and Explanation

Step 1: Coordinates of P and Q: Ellipse: \( \frac{x^2}{3^2} + \frac{y^2}{2^2} = 1 \). Auxiliary Circle: \( x^2 + y^2 = 3^2 = 9 \). Let \( P = (3\cos\theta, 2\sin\theta) \). Since Q lies on the auxiliary circle and PQ is perpendicular to the major axis (same x-coordinate), \( Q = (3\cos\theta, 3\sin\theta) \).
Step 2: Equations of Normals: Normal to Ellipse at P: \[ \frac{ax}{\cos\theta} - \frac{by}{\sin\theta} = a^2 - b^2 \] Substitute \( a=3, b=2 \): \[ \frac{3x}{\cos\theta} - \frac{2y}{\sin\theta} = 9 - 4 = 5 \quad \dots(1) \] Normal to Circle at Q passes through the origin (0,0): Slope \( = \frac{3\sin\theta - 0}{3\cos\theta - 0} = \tan\theta \). Equation is \( y = x\tan\theta \), or \( \sin\theta \cdot x - \cos\theta \cdot y = 0 \). From this, \( \tan\theta = \frac{y}{x} \). Let R be \( (h,k) \), so \( \tan\theta = \frac{k}{h} \). From a right triangle with sides \( h, k \), we get \( \sin\theta = \frac{k}{\sqrt{h^2+k^2}} \) and \( \cos\theta = \frac{h}{\sqrt{h^2+k^2}} \).
Step 3: Find Locus of R: Substitute \( \sin\theta \) and \( \cos\theta \) into equation (1) for point R(h,k): \[ \frac{3h}{h/\sqrt{h^2+k^2}} - \frac{2k}{k/\sqrt{h^2+k^2}} = 5 \] \[ 3\sqrt{h^2+k^2} - 2\sqrt{h^2+k^2} = 5 \] \[ \sqrt{h^2+k^2} = 5 \] Squaring both sides: \[ h^2 + k^2 = 25 \] Locus is \( x^2 + y^2 = 25 \).
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