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Parabola question

2015 · Shift 2 · Q31
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Parabola question

2015 · Shift 2 · Q31

JEE AdvancedMathematicsParabolaNumerical+4 / −1
Suppose that the foci of the ellipse x29+y25=1{{{x^2}} \over 9} + {{{y^2}} \over 5} = 19x2​+5y2​=1 are (f1,0)\left( {{f_1},0} \right)(f1​,0) and (f2,0)\left( {{f_2},0} \right)(f2​,0) where f1>0{{f_1} \gt 0}f1​>0 and f2<0{{f_2} \lt 0}f2​<0. Let P1{P_1}P1​ and P2{P_2}P2​ be two parabolas with a common vertex at (0,0)(0,0)(0,0) and with foci at (f1,0)\left( {{f_1},0} \right)(f1​,0) and (2f2,0)\left( 2{{f_2},0} \right)(2f2​,0), respectively. Let T1{T_1}T1​ be a tangent to P1{P_1}P1​ which passes through (2f2,0)\left( 2{{f_2},0} \right)(2f2​,0) and T2{T_2}T2​ be a tangent to P2{P_2}P2​ which passes through (f1,0)\left( {{f_1},0} \right)(f1​,0). If m1{m_1}m1​ is the slope of T1{T_1}T1​ and m2{m_2}m2​ is the slope of T2{T_2}T2​, then the value of (1m12+m22)\left( {{1 \over {m_1^2}} + m_2^2} \right)(m12​1​+m22​) is
Numerical answer
View written solutionFree

Correct answer: 4

  1. Find the foci of the ellipse

The ellipse is x29+y25=1\frac{x^2}{9}+\frac{y^2}{5}=19x2​+5y2​=1 So, a2=9,b2=5a^2=9,\quad b^2=5a2=9,b2=5 Hence, c2=a2−b2=9−5=4  ⟹  c=2c^2=a^2-b^2=9-5=4 \implies c=2c2=a2−b2=9−5=4⟹c=2 Therefore the foci are at (±2,0)(\pm 2,0)(±2,0).

Given: f1>0,  f2<0f_1>0,\; f_2<0f1​>0,f2​<0 So, f1=2,f2=−2f_1=2,\qquad f_2=-2f1​=2,f2​=−2


  1. Write equations of the two parabolas

Parabola P1P_1P1​

It has vertex at (0,0)(0,0)(0,0) and focus at (f1,0)=(2,0)(f_1,0)=(2,0)(f1​,0)=(2,0). A parabola with vertex at origin and focus (a,0)(a,0)(a,0) has equation y2=4axy^2=4axy2=4ax Here a=2a=2a=2, so P1:y2=8xP_1: y^2=8xP1​:y2=8x

Parabola P2P_2P2​

It has vertex at (0,0)(0,0)(0,0) and focus at (2f2,0)=(2⋅−2,0)=(−4,0)(2f_2,0)=(2\cdot -2,0)=(-4,0)(2f2​,0)=(2⋅−2,0)=(−4,0). A parabola with vertex at origin and focus (−a,0)(-a,0)(−a,0) has equation y2=−4axy^2=-4axy2=−4ax Here a=4a=4a=4, so P2:y2=−16xP_2: y^2=-16xP2​:y2=−16x


  1. Find slope m1m_1m1​ of tangent T1T_1T1​ to P1P_1P1​ passing through (2f2,0)=(−4,0)(2f_2,0)=(-4,0)(2f2​,0)=(−4,0)

For parabola y2=4axy^2=4axy2=4ax A tangent with slope mmm is y=mx+amy=mx+\frac{a}{m}y=mx+ma​

For P1:y2=8xP_1: y^2=8xP1​:y2=8x, we have 4a=8⇒a=24a=8\Rightarrow a=24a=8⇒a=2. So tangent is y=mx+2my=mx+\frac{2}{m}y=mx+m2​

Since it passes through (−4,0)(-4,0)(−4,0), 0=m(−4)+2m0=m(-4)+\frac{2}{m}0=m(−4)+m2​ Multiply by mmm: −4m2+2=0-4m^2+2=0−4m2+2=0 4m2=24m^2=24m2=2 m2=12m^2=\frac12m2=21​ Thus, 1m12=2\frac{1}{m_1^2}=2m12​1​=2


  1. Find slope m2m_2m2​ of tangent T2T_2T2​ to P2P_2P2​ passing through (f1,0)=(2,0)(f_1,0)=(2,0)(f1​,0)=(2,0)

For parabola y2=−4axy^2=-4axy2=−4ax A tangent with slope mmm is y=mx−amy=mx-\frac{a}{m}y=mx−ma​

For P2:y2=−16xP_2: y^2=-16xP2​:y2=−16x, we have 4a=16⇒a=44a=16\Rightarrow a=44a=16⇒a=4. So tangent is y=mx−4my=mx-\frac{4}{m}y=mx−m4​

Since it passes through (2,0)(2,0)(2,0), 0=2m−4m0=2m-\frac{4}{m}0=2m−m4​ Multiply by mmm: 2m2−4=02m^2-4=02m2−4=0 m2=2m^2=2m2=2 Thus, m22=2m_2^2=2m22​=2


  1. Compute the required value

We need 1m12+m22\frac{1}{m_1^2}+m_2^2m12​1​+m22​ Substitute: 1m12=2,m22=2\frac{1}{m_1^2}=2,\qquad m_2^2=2m12​1​=2,m22​=2 Therefore, 1m12+m22=2+2=4\frac{1}{m_1^2}+m_2^2=2+2=4m12​1​+m22​=2+2=4


  1. Comparison with stored answer

Derived answer = 444. Stored correct answer = 444. They agree.

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