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Hyperbola question

2022 · 26 Jul · Shift 2 · Q30
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  5. /2022 · 26 Jul · Shift 2 · Q30

Hyperbola question

2022 · 26 Jul · Shift 2 · Q30

JEE MainMathematicsHyperbolaMCQ+4 / −1
If the line x−1=0x-1=0x−1=0 is a directrix of the hyperbola kx2−y2=6k x^{2}-y^{2}=6kx2−y2=6, then the hyperbola passes through the point :
  1. A
    (−25,6)(-2 \sqrt{5}, 6)(−25​,6)
  2. B
    (−5,3)(-\sqrt{5}, 3)(−5​,3)
  3. C
    (5,−2)(\sqrt{5},-2)(5​,−2)
  4. D
    (25,36)(2 \sqrt{5}, 3 \sqrt{6})(25​,36​)
View written solutionFree

Correct answer: C

  1. Write the hyperbola in standard form

Given: kx2−y2=6kx^2-y^2=6kx2−y2=6

Divide by 666: kx26−y26=1\frac{kx^2}{6}-\frac{y^2}{6}=16kx2​−6y2​=1

Rewrite as x26/k−y26=1\frac{x^2}{6/k}-\frac{y^2}{6}=16/kx2​−6y2​=1

So this is a hyperbola of the form x2a2−y2b2=1\frac{x^2}{a^2}-\frac{y^2}{b^2}=1a2x2​−b2y2​=1 with a2=6k,b2=6a^2=\frac{6}{k}, \qquad b^2=6a2=k6​,b2=6

  1. Use the directrix condition

For the hyperbola x2a2−y2b2=1\frac{x^2}{a^2}-\frac{y^2}{b^2}=1a2x2​−b2y2​=1 the directrices are x=±aex=\pm \frac{a}{e}x=±ea​ where e=1+b2a2e=\sqrt{1+\frac{b^2}{a^2}}e=1+a2b2​​

Given directrix: x−1=0⇒x=1x-1=0 \Rightarrow x=1x−1=0⇒x=1

Hence, ae=1⇒a=e\frac{a}{e}=1 \Rightarrow a=eea​=1⇒a=e

Now, e2=1+b2a2e^2=1+\frac{b^2}{a^2}e2=1+a2b2​ Since a=ea=ea=e, we get a2=1+b2a2a^2=1+\frac{b^2}{a^2}a2=1+a2b2​

Using b2=6b^2=6b2=6: a2=1+6a2a^2=1+\frac{6}{a^2}a2=1+a26​

Multiply by a2a^2a2: a4−a2−6=0a^4-a^2-6=0a4−a2−6=0

Let t=a2t=a^2t=a2. Then t2−t−6=0t^2-t-6=0t2−t−6=0 (t−3)(t+2)=0 (t-3)(t+2)=0(t−3)(t+2)=0

Since a2>0a^2>0a2>0, we take a2=3a^2=3a2=3

Thus, 6k=3⇒k=2\frac{6}{k}=3 \Rightarrow k=2k6​=3⇒k=2

  1. Equation of hyperbola

Substitute k=2k=2k=2 into the original equation: 2x2−y2=62x^2-y^2=62x2−y2=6

  1. Check the options

We test each point in 2x2−y2=62x^2-y^2=62x2−y2=6

  • A: (−25,6)(-2\sqrt{5},6)(−25​,6) 2(20)−36=40−36=4≠62(20)-36=40-36=4 \ne 62(20)−36=40−36=4=6 Not on the hyperbola.

  • B: (−5,3)(-\sqrt{5},3)(−5​,3) 2(5)−9=10−9=1≠62(5)-9=10-9=1 \ne 62(5)−9=10−9=1=6 Not on the hyperbola.

  • C: (5,−2)(\sqrt{5},-2)(5​,−2) 2(5)−(−2)2=10−4=62(5)-(-2)^2=10-4=62(5)−(−2)2=10−4=6 Lies on the hyperbola.

  • D: (25,36)(2\sqrt{5},3\sqrt{6})(25​,36​) 2(20)−(36)2=40−54=−14≠62(20)-(3\sqrt{6})^2=40-54=-14 \ne 62(20)−(36​)2=40−54=−14=6 Not on the hyperbola.

  1. Conclusion

The hyperbola passes through (5,−2)\boxed{(\sqrt{5},-2)}(5​,−2)​ which is Option C.

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