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Hyperbola question

2022 · 28 Jul · Shift 2 · Q32
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  5. /2022 · 28 Jul · Shift 2 · Q32

Hyperbola question

2022 · 28 Jul · Shift 2 · Q32

JEE MainMathematicsHyperbolaMCQ+4 / −1
Let the hyperbola H:x2a2−y2b2=1H: \frac{x^{2}}{a^{2}}-\frac{y^{2}}{b^{2}}=1H:a2x2​−b2y2​=1 pass through the point (22,−22)(2 \sqrt{2},-2 \sqrt{2})(22​,−22​). A parabola is drawn whose focus is same as the focus of H\mathrm{H}H with positive abscissa and the directrix of the parabola passes through the other focus of H\mathrm{H}H. If the length of the latus rectum of the parabola is e times the length of the latus rectum of H\mathrm{H}H, where e is the eccentricity of H, then which of the following points lies on the parabola?
  1. A
    (23,32)(2 \sqrt{3}, 3 \sqrt{2})(23​,32​)
  2. B
    (33,−62)\mathbf(3 \sqrt{3},-6 \sqrt{2})(33​,−62​)
  3. C
    (3,−6)(\sqrt{3},-\sqrt{6})(3​,−6​)
  4. D
    (36,62)(3 \sqrt{6}, 6 \sqrt{2})(36​,62​)
View written solutionFree

Correct answer: B

  1. Hyperbola data

Given H:x2a2−y2b2=1H:\frac{x^2}{a^2}-\frac{y^2}{b^2}=1H:a2x2​−b2y2​=1 and it passes through (22,−22)(2\sqrt2,-2\sqrt2)(22​,−22​).

So, (22)2a2−(−22)2b2=1\frac{(2\sqrt2)^2}{a^2}-\frac{(-2\sqrt2)^2}{b^2}=1a2(22​)2​−b2(−22​)2​=1 8a2−8b2=1\frac{8}{a^2}-\frac{8}{b^2}=1a28​−b28​=1 8(1a2−1b2)=1(Equation 1)8\left(\frac{1}{a^2}-\frac{1}{b^2}\right)=1 \quad \text{(Equation 1)}8(a21​−b21​)=1(Equation 1)

For the hyperbola, eccentricity is e=ca,c2=a2+b2e=\frac{c}{a}, \qquad c^2=a^2+b^2e=ac​,c2=a2+b2

Length of latus rectum of hyperbola: LH=2b2aL_H=\frac{2b^2}{a}LH​=a2b2​

  1. Parabola construction

The focus of the parabola is the focus of HHH with positive abscissa, so F=(c,0)F=(c,0)F=(c,0) The other focus of HHH is (−c,0)(-c,0)(−c,0).

Let the directrix of parabola be the vertical line x=dx=dx=d Since it passes through (−c,0)(-c,0)(−c,0), we get d=−cd=-cd=−c

Thus parabola has focus (c,0)(c,0)(c,0) and directrix x=−cx=-cx=−c. Its axis is the xxx-axis, and vertex is midpoint between focus and directrix: (c+(−c)2,0)=(0,0)\left(\frac{c+(-c)}{2},0\right)=(0,0)(2c+(−c)​,0)=(0,0) So the parabola is y2=4cxy^2=4cxy2=4cx with parameter ccc.

Length of latus rectum of parabola: LP=4cL_P=4cLP​=4c

  1. Using the latus rectum condition

Given: LP=e⋅LHL_P=e\cdot L_HLP​=e⋅LH​ So, 4c=e⋅2b2a4c=e\cdot \frac{2b^2}{a}4c=e⋅a2b2​ Using e=cae=\frac{c}{a}e=ac​, 4c=ca⋅2b2a4c=\frac{c}{a}\cdot \frac{2b^2}{a}4c=ac​⋅a2b2​ Assuming c≠0c\neq 0c=0, divide by ccc: 4=2b2a24=\frac{2b^2}{a^2}4=a22b2​ b2=2a2b^2=2a^2b2=2a2

  1. Find a,b,ca,b,ca,b,c

Substitute b2=2a2b^2=2a^2b2=2a2 into Equation 1: 8(1a2−12a2)=18\left(\frac{1}{a^2}-\frac{1}{2a^2}\right)=18(a21​−2a21​)=1 8(12a2)=18\left(\frac{1}{2a^2}\right)=18(2a21​)=1 4a2=1\frac{4}{a^2}=1a24​=1 a2=4a^2=4a2=4 Then b2=8b^2=8b2=8 And c2=a2+b2=4+8=12c^2=a^2+b^2=4+8=12c2=a2+b2=4+8=12 c=23c=2\sqrt3c=23​

Hence the parabola is y2=4cx=83 xy^2=4cx=8\sqrt3\,xy2=4cx=83​x

  1. Check the options

We test each point in y2=83 xy^2=8\sqrt3\,xy2=83​x

Option A: (23,32)(2\sqrt3,3\sqrt2)(23​,32​)

y2=(32)2=18y^2=(3\sqrt2)^2=18y2=(32​)2=18 83 x=83(23)=8⋅2⋅3=488\sqrt3\,x=8\sqrt3(2\sqrt3)=8\cdot 2\cdot 3=4883​x=83​(23​)=8⋅2⋅3=48 18≠4818\neq 4818=48 So A is not on the parabola.

Option B: (33,−62)(3\sqrt3,-6\sqrt2)(33​,−62​)

y2=(−62)2=72y^2=(-6\sqrt2)^2=72y2=(−62​)2=72 83 x=83(33)=8⋅3⋅3=728\sqrt3\,x=8\sqrt3(3\sqrt3)=8\cdot 3\cdot 3=7283​x=83​(33​)=8⋅3⋅3=72 72=7272=7272=72 So B lies on the parabola.

Option C: (3,−6)(\sqrt3,-\sqrt6)(3​,−6​)

y2=(−6)2=6y^2=(-\sqrt6)^2=6y2=(−6​)2=6 83 x=83(3)=248\sqrt3\,x=8\sqrt3(\sqrt3)=2483​x=83​(3​)=24 6≠246\neq 246=24 So C is not on the parabola.

Option D: (36,62)(3\sqrt6,6\sqrt2)(36​,62​)

y2=(62)2=72y^2=(6\sqrt2)^2=72y2=(62​)2=72 83 x=83(36)=2418=7228\sqrt3\,x=8\sqrt3(3\sqrt6)=24\sqrt{18}=72\sqrt283​x=83​(36​)=2418​=722​ 72≠72272\neq 72\sqrt272=722​ So D is not on the parabola.

  1. Conclusion

The point lying on the parabola is: (33,−62)\boxed{(3\sqrt3,-6\sqrt2)}(33​,−62​)​ which is Option B.

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