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Hyperbola question

2025 · 3 Apr · Shift 1 · Q48
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Hyperbola question

2025 · 3 Apr · Shift 1 · Q48

JEE MainMathematicsHyperbolaNumerical+4 / −1
Let the product of the focal distances of the point P(4,23)\mathbf{P}(4,2 \sqrt{3})P(4,23​) on the hyperbola H:x2a2−y2b2=1\mathrm{H}: \frac{x^2}{a^2}-\frac{y^2}{b^2}=1H:a2x2​−b2y2​=1 be 32 . Let the length of the conjugate axis of H be ppp and the length of its latus rectum be qqq. Then p2+q2p^2+q^2p2+q2 is equal to ‾\underline{\hspace{2cm}}​
Numerical answer
View written solutionFree

Correct answer: 120

  1. Given hyperbola and point on it

We have x2a2−y2b2=1\frac{x^2}{a^2}-\frac{y^2}{b^2}=1a2x2​−b2y2​=1 and the point P(4,23)P(4,2\sqrt{3})P(4,23​) lies on it.

So, 16a2−12b2=1...(1)\frac{16}{a^2}-\frac{12}{b^2}=1 \qquad ...(1)a216​−b212​=1...(1)


  1. Use the product of focal distances

For the hyperbola x2a2−y2b2=1,\frac{x^2}{a^2}-\frac{y^2}{b^2}=1,a2x2​−b2y2​=1, the foci are at F1(−c,0),F2(c,0),F_1(-c,0),\quad F_2(c,0),F1​(−c,0),F2​(c,0), where c2=a2+b2.c^2=a^2+b^2.c2=a2+b2.

If the distances of P(x,y)P(x,y)P(x,y) from the foci are r1r_1r1​ and r2r_2r2​, then r1r2=((x+c)2+y2)((x−c)2+y2).r_1r_2=\sqrt{\big((x+c)^2+y^2\big)\big((x-c)^2+y^2\big)}.r1​r2​=((x+c)2+y2)((x−c)2+y2)​.

For P(4,23)P(4,2\sqrt3)P(4,23​), r1r2=((4+c)2+12)((4−c)2+12)=32.r_1r_2=\sqrt{\big((4+c)^2+12\big)\big((4-c)^2+12\big)}=32.r1​r2​=((4+c)2+12)((4−c)2+12)​=32.

Squaring, ((4+c)2+12)((4−c)2+12)=1024.\big((4+c)^2+12\big)\big((4-c)^2+12\big)=1024.((4+c)2+12)((4−c)2+12)=1024.

Now, ((4+c)2+12)=c2+8c+28,((4+c)^2+12)=c^2+8c+28,((4+c)2+12)=c2+8c+28, ((4−c)2+12)=c2−8c+28.((4-c)^2+12)=c^2-8c+28.((4−c)2+12)=c2−8c+28.

Hence (c2+28+8c)(c2+28−8c)=1024,(c^2+28+8c)(c^2+28-8c)=1024,(c2+28+8c)(c2+28−8c)=1024, (c2+28)2−64c2=1024. (c^2+28)^2-64c^2=1024.(c2+28)2−64c2=1024.

So, c4−8c2+784=1024,c^4-8c^2+784=1024,c4−8c2+784=1024, c4−8c2−240=0.c^4-8c^2-240=0.c4−8c2−240=0.

Let u=c2u=c^2u=c2. Then u2−8u−240=0,u^2-8u-240=0,u2−8u−240=0, u=20 or −12.u=20 \text{ or } -12.u=20 or −12.

Since c2>0c^2>0c2>0, c2=20.c^2=20.c2=20.

Thus, a2+b2=20....(2)a^2+b^2=20. \qquad ...(2)a2+b2=20....(2)


  1. Find a2a^2a2 and b2b^2b2

Let A=a2,B=b2.A=a^2,\quad B=b^2.A=a2,B=b2. Then from (2), A+B=20....(3)A+B=20. \qquad ...(3)A+B=20....(3)

From (1), 16A−12B=1.\frac{16}{A}-\frac{12}{B}=1.A16​−B12​=1.

Using B=20−AB=20-AB=20−A, 16A−1220−A=1.\frac{16}{A}-\frac{12}{20-A}=1.A16​−20−A12​=1.

Multiply by A(20−A)A(20-A)A(20−A): 16(20−A)−12A=A(20−A).16(20-A)-12A=A(20-A).16(20−A)−12A=A(20−A).

So, 320−16A−12A=20A−A2,320-16A-12A=20A-A^2,320−16A−12A=20A−A2, 320−28A=20A−A2,320-28A=20A-A^2,320−28A=20A−A2, A2−48A+320=0.A^2-48A+320=0.A2−48A+320=0.

Thus, A=40 or 8.A=40 \text{ or } 8.A=40 or 8.

Since A+B=20A+B=20A+B=20, A=40A=40A=40 is impossible. Hence a2=8,a^2=8,a2=8, b2=12.b^2=12.b2=12.


  1. Compute ppp and qqq
  • Length of conjugate axis of the hyperbola is p=2b=212.p=2b=2\sqrt{12}.p=2b=212​. So, p2=4b2=4⋅12=48.p^2=4b^2=4\cdot 12=48.p2=4b2=4⋅12=48.

  • Length of latus rectum of the hyperbola is q=2b2a=2⋅128.q=\frac{2b^2}{a}=\frac{2\cdot 12}{\sqrt8}.q=a2b2​=8​2⋅12​. But we need only q2q^2q2: q2=(2b2a)2=4b4a2.q^2=\left(\frac{2b^2}{a}\right)^2=\frac{4b^4}{a^2}.q2=(a2b2​)2=a24b4​. Now, b4=122=144,a2=8,b^4=12^2=144, \quad a^2=8,b4=122=144,a2=8, so q2=4⋅1448=72.q^2=\frac{4\cdot 144}{8}=72.q2=84⋅144​=72.

Therefore, p2+q2=48+72=120.p^2+q^2=48+72=120.p2+q2=48+72=120.


  1. Final answer

120\boxed{120}120​

This matches the stored correct answer.

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