Skip to main content
Back to Conic Sections
JEE Main 2023
Conic Sections
Hyperbola
Hard

Question

Let H:x2a2+y2b2=1H: \frac{-x^2}{a^2}+\frac{y^2}{b^2}=1 be the hyperbola, whose eccentricity is 3\sqrt{3} and the length of the latus rectum is 434 \sqrt{3}. Suppose the point (α,6),α>0(\alpha, 6), \alpha>0 lies on HH. If β\beta is the product of the focal distances of the point (α,6)(\alpha, 6), then α2+β\alpha^2+\beta is equal to

Options

Solution

This solution will guide you through the problem step-by-step, explaining the underlying concepts and formulas for hyperbolas.


1. Understanding the Hyperbola and its Standard Form

The given equation of the hyperbola is H:x2a2+y2b2=1H: \frac{-x^2}{a^2}+\frac{y^2}{b^2}=1. The first crucial step in any conic section problem is to identify its standard form and orientation. We can rearrange the terms to get: y2b2x2a2=1\frac{y^2}{b^2} - \frac{x^2}{a^2} = 1 Key Concept: This is the standard form of a hyperbola where the transverse axis lies along the y-axis. This means the major features are oriented vertically:

  • The vertices are at (0,±b)(0, \pm b).
  • The foci are at (0,±be)(0, \pm be).
  • The directrices are y=±bey = \pm \frac{b}{e}.
  • The relationship between the semi-conjugate axis (aa), semi-transverse axis (bb), and eccentricity (ee) is given by a2=b2(e21)a^2 = b^2(e^2-1).
  • The length of the latus rectum (LRLR) is 2a2b\frac{2a^2}{b}.

Common Mistake: Do not confuse this with a hyperbola whose transverse axis is along the x-axis (x2a2y2b2=1\frac{x^2}{a^2} - \frac{y^2}{b^2} = 1). For that type, the roles of aa and bb are swapped in the eccentricity relation (b2=a2(e21)b^2 = a^2(e^2-1)) and latus rectum formula (LR=2b2aLR = \frac{2b^2}{a}), and the foci/vertices are on the x-axis. Always check which term is positive in the standard equation to determine the transverse axis.


2. Determining the Hyperbola Parameters 'a' and 'b'

We are given two properties of the hyperbola:

  1. Eccentricity, e=3e = \sqrt{3}.
  2. Length of the latus rectum, LR=43LR = 4\sqrt{3}.

We will use the formulas specific to a hyperbola with its transverse axis along the y-axis.

Step 2a: Using the Eccentricity to relate 'a' and 'b' The relationship for eccentricity is a2=b2(e21)a^2 = b^2(e^2-1). Substitute the given value e=3e = \sqrt{3}: a2=b2((3)21)a^2 = b^2((\sqrt{3})^2 - 1) a2=b2(31)a^2 = b^2(3 - 1) a2=2b2(Equation 1)a^2 = 2b^2 \quad \text{(Equation 1)} Explanation: This equation links the semi-conjugate axis (aa) and semi-transverse axis (bb) through the eccentricity. It's a fundamental property that defines the "shape" or "openness" of the hyperbola.

Step 2b: Using the Length of the Latus Rectum to relate 'a' and 'b' The formula for the length of the latus rectum is LR=2a2bLR = \frac{2a^2}{b}. Substitute the given value LR=43LR = 4\sqrt{3}: 43=2a2b4\sqrt{3} = \frac{2a^2}{b} Divide both sides by 2: 23=a2b2\sqrt{3} = \frac{a^2}{b} a2=23b(Equation 2)a^2 = 2\sqrt{3}b \quad \text{(Equation 2)} Explanation: The latus rectum is a chord passing through a focus and perpendicular to the transverse axis. Its length provides another geometric constraint on the dimensions of the hyperbola.

Step 2c: Solving for 'a' and 'b' Now we have a system of two equations with two unknowns (a2a^2 and bb):

  1. a2=2b2a^2 = 2b^2
  2. a2=23ba^2 = 2\sqrt{3}b

Equating the expressions for a2a^2 from both equations: 2b2=23b2b^2 = 2\sqrt{3}b Since bb represents a length

Practice More Conic Sections Questions

View All Questions