JEE Challenger
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Area of Triangle Formed by Hyperbola Vertex Focus and Latus Rectum

Consider a branch of the hyperbola
x2−2y2−22x−42y−6=0x^2 - 2y^2 - 2\sqrt{2}x - 4\sqrt{2}y - 6 = 0
with vertex at the point AA. Let BB be one of the end points of its latus rectum. If CC is the focus of the hyperbola nearest to the point AA, then the area of the triangle ABCABC is

Options

A

1−231 - \sqrt{\frac{2}{3}}

B

32−1\sqrt{\frac{3}{2}} - 1

Correct
C

1+231 + \sqrt{\frac{2}{3}}

D

32+1\sqrt{\frac{3}{2}} + 1

Topics & Concepts

Step-by-Step Solution

To find the area of the triangle ABCABC, we first rewrite the equation of the given hyperbola in standard form:

x2−2y2−22x−42y−6=0x^2 - 2y^2 - 2\sqrt{2}x - 4\sqrt{2}y - 6 = 0

Group the xx and yy terms and complete the squares:
(x2−22x)−2(y2+22y)=6(x^2 - 2\sqrt{2}x) - 2(y^2 + 2\sqrt{2}y) = 6
((x−2)2−2)−2((y+2)2−2)=6\left((x - \sqrt{2})^2 - 2\right) - 2\left((y + \sqrt{2})^2 - 2\right) = 6
(x−2)2−2−2(y+2)2+4=6(x - \sqrt{2})^2 - 2 - 2(y + \sqrt{2})^2 + 4 = 6
(x−2)2−2(y+2)2=4(x - \sqrt{2})^2 - 2(y + \sqrt{2})^2 = 4

Dividing both sides by 44, we get the standard form of the hyperbola:
(x−2)24−(y+2)22=1\frac{(x - \sqrt{2})^2}{4} - \frac{(y + \sqrt{2})^2}{2} = 1

Let X=x−2X = x - \sqrt{2} and Y=y+2Y = y + \sqrt{2}. The equation becomes:
X2a2−Y2b2=1\frac{X^2}{a^2} - \frac{Y^2}{b^2} = 1

where:
a2=4  ⟹  a=2a^2 = 4 \implies a = 2
b2=2  ⟹  b=2b^2 = 2 \implies b = \sqrt{2}

The eccentricity ee of the hyperbola is:
e=1+b2a2=1+24=32e = \sqrt{1 + \frac{b^2}{a^2}} = \sqrt{1 + \frac{2}{4}} = \sqrt{\frac{3}{2}}

In the (X,Y)(X, Y) coordinate system, the coordinates of the key points are:

  1. Vertex (AA): Lies on the transverse axis at (a,0)=(2,0)(a, 0) = (2, 0).
  2. Focus (CC): The focus nearest to AA is located at (ae,0)=(232,0)(ae, 0) = \left(2\sqrt{\frac{3}{2}}, 0\right).
  3. Endpoint of Latus Rectum (BB): The latus rectum passes through the focus CC perpendicular to the transverse axis. Thus, BB has coordinates (ae,b2a)=(232,1)\left(ae, \frac{b^2}{a}\right) = \left(2\sqrt{\frac{3}{2}}, 1\right).

Since AA and CC both lie on the XX-axis (Y=0Y = 0), and BB and CC share the same XX-coordinate (X=aeX = ae), the line segment BCBC is perpendicular to ACAC. Therefore, △ABC\triangle ABC is a right-angled triangle with the right angle at CC.

The length of the base ACAC is:
AC=ae−a=a(e−1)=2(32−1)AC = ae - a = a(e - 1) = 2\left(\sqrt{\frac{3}{2}} - 1\right)

The length of the height BCBC is:
BC=b2a=22=1BC = \frac{b^2}{a} = \frac{2}{2} = 1

The area of △ABC\triangle ABC is given by:
Area=12×base×height=12×AC×BC\text{Area} = \frac{1}{2} \times \text{base} \times \text{height} = \frac{1}{2} \times AC \times BC
Area=12×2(32−1)×1=32−1\text{Area} = \frac{1}{2} \times 2\left(\sqrt{\frac{3}{2}} - 1\right) \times 1 = \sqrt{\frac{3}{2}} - 1

Thus, the correct option is B.