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Hyperbola question

2008 · Shift 2 · Q36
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Hyperbola question

2008 · Shift 2 · Q36

JEE AdvancedMathematicsHyperbolaMCQ+3 / −1
Consider a branch of the hyperbola x2−2y2−22x−42y−6=0{x^2} - 2{y^2} - 2\sqrt 2 x - 4\sqrt 2 y - 6 = 0x2−2y2−22​x−42​y−6=0 with vertex at the point AAA. Let BBB be one of the end points of its latus rectum. If CCC is the focus of the hyperbola nearest to the point AAA, then the area of the triangle ABCABCABC is
  1. A
    1−231 - \sqrt {{2 \over 3}}1−32​​
  2. B
    32−1\sqrt {{3 \over 2}} - 123​​−1
  3. C
    1+231 + \sqrt {{2 \over 3}}1+32​​
  4. D
    32+1\sqrt {{3 \over 2}} + 123​​+1
View written solutionFree

Correct answer: B

Step 1: Convert the equation of the hyperbola to standard form.

The given equation is x2−2y2−22x−42y−6=0{x^2} - 2{y^2} - 2 \sqrt 2 x - 4\sqrt 2 y - 6 = 0x2−2y2−22​x−42​y−6=0. To convert this to the standard form, we complete the square for the x and y terms.

Group the terms: (x2−22x)−2(y2+22y)−6=0(x^2 - 2\sqrt{2}x) - 2(y^2 + 2\sqrt{2}y) - 6 = 0(x2−22​x)−2(y2+22​y)−6=0

Complete the square for x: (x2−22x+(2)2−(2)2)=(x−2)2−2(x^2 - 2\sqrt{2}x + (\sqrt{2})^2 - (\sqrt{2})^2) = (x - \sqrt{2})^2 - 2(x2−22​x+(2​)2−(2​)2)=(x−2​)2−2

Complete the square for y: −2(y2+22y+(2)2−(2)2)=−2((y+2)2−2)=−2(y+2)2+4-2(y^2 + 2\sqrt{2}y + (\sqrt{2})^2 - (\sqrt{2})^2) = -2((y + \sqrt{2})^2 - 2) = -2(y + \sqrt{2})^2 + 4−2(y2+22​y+(2​)2−(2​)2)=−2((y+2​)2−2)=−2(y+2​)2+4

Substitute these back into the equation: ((x−2)2−2)+(−2(y+2)2+4)−6=0((x - \sqrt{2})^2 - 2) + (-2(y + \sqrt{2})^2 + 4) - 6 = 0((x−2​)2−2)+(−2(y+2​)2+4)−6=0 (x−2)2−2(y+2)2−4=0(x - \sqrt{2})^2 - 2(y + \sqrt{2})^2 - 4 = 0(x−2​)2−2(y+2​)2−4=0 (x−2)2−2(y+2)2=4(x - \sqrt{2})^2 - 2(y + \sqrt{2})^2 = 4(x−2​)2−2(y+2​)2=4

Divide by 4 to get the standard form (x−h)2a2−(y−k)2b2=1\frac{(x-h)^2}{a^2} - \frac{(y-k)^2}{b^2} = 1a2(x−h)2​−b2(y−k)2​=1: (x−2)24−(y+2)22=1\frac{(x - \sqrt{2})^2}{4} - \frac{(y + \sqrt{2})^2}{2} = 14(x−2​)2​−2(y+2​)2​=1

Step 2: Identify the parameters of the hyperbola.

From the standard form, we can identify:

  • Center (h,k)=(2,−2)(h, k) = (\sqrt{2}, -\sqrt{2})(h,k)=(2​,−2​)
  • a2=4  ⟹  a=2a^2 = 4 \implies a = 2a2=4⟹a=2
  • b2=2  ⟹  b=2b^2 = 2 \implies b = \sqrt{2}b2=2⟹b=2​

Now, we calculate the eccentricity, eee: e=1+b2a2=1+24=1+12=32e = \sqrt{1 + \frac{b^2}{a^2}} = \sqrt{1 + \frac{2}{4}} = \sqrt{1 + \frac{1}{2}} = \sqrt{\frac{3}{2}}e=1+a2b2​​=1+42​​=1+21​​=23​​

Step 3: Determine the coordinates of the points A, B, and C.

  • Point A (Vertex): The vertices of the hyperbola are at (h±a,k)(h \pm a, k)(h±a,k). We are considering one branch. Let's take the right branch. The vertex AAA is: A=(h+a,k)=(2+2,−2)A = (h+a, k) = (\sqrt{2} + 2, -\sqrt{2})A=(h+a,k)=(2​+2,−2​)

  • Point C (Focus nearest to A): The foci are at (h±ae,k)(h \pm ae, k)(h±ae,k). First, calculate aeaeae: ae=2⋅32=4⋅32=6ae = 2 \cdot \sqrt{\frac{3}{2}} = \sqrt{4 \cdot \frac{3}{2}} = \sqrt{6}ae=2⋅23​​=4⋅23​​=6​. The foci are S1=(2+6,−2)S_1 = (\sqrt{2} + \sqrt{6}, -\sqrt{2})S1​=(2​+6​,−2​) and S2=(2−6,−2)S_2 = (\sqrt{2} - \sqrt{6}, -\sqrt{2})S2​=(2​−6​,−2​). The vertex A is on the right branch, so the nearest focus will also be on the right side of the center. Thus, C is S1S_1S1​. C=(2+6,−2)C = (\sqrt{2} + \sqrt{6}, -\sqrt{2})C=(2​+6​,−2​)

  • Point B (Endpoint of Latus Rectum): The latus rectum is a line segment passing through a focus and perpendicular to the transverse axis. Its endpoints are (h±ae,k±b2a)(h \pm ae, k \pm \frac{b^2}{a})(h±ae,k±ab2​). Since C is the focus, the endpoints of the latus rectum through C have coordinates (xC,yC±b2a)(x_C, y_C \pm \frac{b^2}{a})(xC​,yC​±ab2​). Calculate the semi-latus rectum length: b2a=22=1\frac{b^2}{a} = \frac{2}{2} = 1ab2​=22​=1. The endpoints are (2+6,−2±1)(\sqrt{2} + \sqrt{6}, -\sqrt{2} \pm 1)(2​+6​,−2​±1). Let's choose one of these points as B: B=(2+6,−2+1)B = (\sqrt{2} + \sqrt{6}, -\sqrt{2} + 1)B=(2​+6​,−2​+1)

Step 4: Calculate the area of triangle ABC.

The coordinates of the vertices of the triangle are: A=(2+2,−2)A = (\sqrt{2} + 2, -\sqrt{2})A=(2​+2,−2​) B=(2+6,−2+1)B = (\sqrt{2} + \sqrt{6}, -\sqrt{2} + 1)B=(2​+6​,−2​+1) C=(2+6,−2)C = (\sqrt{2} + \sqrt{6}, -\sqrt{2})C=(2​+6​,−2​)

Notice that points A and C have the same y-coordinate, so the side AC is horizontal. Points B and C have the same x-coordinate, so the side BC is vertical. Therefore, triangle ABC is a right-angled triangle with the right angle at C.

Calculate the lengths of the base AC and the height BC:

  • Length of base AC: AC=∣xC−xA∣=∣(2+6)−(2+2)∣=∣6−2∣=6−2(since 6>2)AC = |x_C - x_A| = |(\sqrt{2} + \sqrt{6}) - (\sqrt{2} + 2)| = |\sqrt{6} - 2| = \sqrt{6} - 2 \quad (\text{since } \sqrt{6} > 2)AC=∣xC​−xA​∣=∣(2​+6​)−(2​+2)∣=∣6​−2∣=6​−2(since 6​>2)
  • Length of height BC: BC=∣yB−yC∣=∣(−2+1)−(−2)∣=∣1∣=1BC = |y_B - y_C| = |(-\sqrt{2} + 1) - (-\sqrt{2})| = |1| = 1BC=∣yB​−yC​∣=∣(−2​+1)−(−2​)∣=∣1∣=1

Now, calculate the area of the triangle: Area(△ABC)=12×base×height=12×AC×BCArea(\triangle ABC) = \frac{1}{2} \times \text{base} \times \text{height} = \frac{1}{2} \times AC \times BCArea(△ABC)=21​×base×height=21​×AC×BC Area=12×(6−2)×1=6−22=62−1Area = \frac{1}{2} \times (\sqrt{6} - 2) \times 1 = \frac{\sqrt{6} - 2}{2} = \frac{\sqrt{6}}{2} - 1Area=21​×(6​−2)×1=26​−2​=26​​−1

To match the options, we can rewrite 62\frac{\sqrt{6}}{2}26​​ as 64=32\sqrt{\frac{6}{4}} = \sqrt{\frac{3}{2}}46​​=23​​. So, the area is 32−1\sqrt{\frac{3}{2}} - 123​​−1.

This matches option B.

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