Question

Let \( P \) be a parabola with vertex \( (2, 3) \) and directrix \( 2x + y = 6 \). Let an ellipse \( E : \frac{x^2}{a^2} + \frac{y^2}{b^2} = 1 \), \( a > b \), of eccentricity \( \frac{1}{\sqrt{2}} \) pass through the focus of the parabola \( P \). Then the square of the length of the latus rectum of \( E \) is

• A. \(\frac{385}{8}\)
• B. \(\frac{347}{8}\)
• C. \(\frac{512}{25}\)
• D. \(\frac{656}{25}\)

Answer 1

The Correct Option is D

Find the focus of the parabola. The equation of the directrix is:

\[ 2x + y = 6 \]

The vertex of the parabola is \( (2, 3) \). The equation of a parabola with vertex \( (h, k) \) and directrix

\[ Ax + By + C = 0 \text{ has focus at: } \left( h + \frac{A}{\sqrt{A^2 + B^2}}, k + \frac{B}{\sqrt{A^2 + B^2}} \right) \]

For our parabola:

\[ A = 2, \quad B = 1, \quad C = -6, \quad h = 2, \quad k = 3 \]

Thus, the distance from the vertex to the directrix is:

\[ \left| \frac{2 \cdot 2 + 1 \cdot 3 - 6}{\sqrt{2^2 + 1^2}} \right| = \left| \frac{4 + 3 - 6}{\sqrt{5}} \right| = \frac{1}{\sqrt{5}} \]

The focus of the parabola \( P \) is at:

\[ \left( 2 + \frac{2}{\sqrt{5}}, 3 + \frac{1}{\sqrt{5}} \right) \]

Use the eccentricity of the ellipse. The eccentricity \( e \) of the ellipse \( E \) is given as \( \frac{1}{\sqrt{2}} \). For an ellipse,

\[ e = \frac{\sqrt{a^2 - b^2}}{a} \]

Squaring both sides:

\[ \frac{1}{2} = \frac{a^2 - b^2}{a^2} \] \[ a^2 - b^2 = \frac{a^2}{2} \] \[ b^2 = \frac{a^2}{2} \]

Calculate the length of the latus rectum. The length of the latus rectum of an ellipse is given by \( \frac{2b^2}{a} \). Substituting \( b^2 = \frac{a^2}{2} \):

\[ \text{Latus Rectum} = \frac{2 \cdot \frac{a^2}{2}}{a} = \frac{a^2}{a} = a \]

Find \( a \) using the focus of the parabola. Since the ellipse passes through the focus of the parabola, substitute the coordinates of the focus into the ellipse equation and solve for \( a \) and \( b \).

After finding \( a \), calculate \( \left( \frac{2b^2}{a} \right)^2 \) to get the square of the latus rectum.

Thus, the answer is:

\[ \frac{656}{25} \]