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JEE Main 20199 AprilEvening Shift
Exam
JEE Main
Year
2019
Date
9 April
Shift
Evening Shift
Subject
Mathematics
Chapter
Ellipse

JEE Main 2019 · 9 April · Evening Shift · Mathematics · Ellipse

If the tangent to the parabola y ^ 2 = x at a point (α, β), (β > 0)...

If the tangent to the parabola y2=x{y}^{2} = x at a point \left(\alpha, \beta\right), \left(\beta > 0\right) is also a tangenttotheellipse,x2+2y2=1to the ellipse,{x}^{2} + 2{y}^{2} = 1 then α\alpha is equal to:

Options

  1. A2−1\sqrt{2} - 1
  2. B22+12\sqrt{2} + 1
  3. C2+1\sqrt{2} + 1
  4. D22−12\sqrt{2} - 1

Correct answer

C. 2+1\sqrt{2} + 1

Step-by-step solution

Since (α,β)\left(\alpha, \beta\right) lies on the parabola ⇒α=β2⇒ \alpha = {\beta}^{2}

⇒(α,β)=(β2,β)⇒ \left(\alpha, \beta\right) = \left({\beta}^{2}, \beta\right)

Tangent to a parabola y2=4ax{y}^{2} = 4ax at a point \left({x}_{1}, {y}_{1}\right)isisy{y}_{1} = 2a\left(x + {x}_{1}\right)</p><p>Therefore,tangenttotheparabola</p><p>Therefore, tangent to the parabola{y}^{2} = xat(β2,β)at \left({\beta}^{2}, \beta\right) isy⋅β=12(x+β2)y ⋅ \beta = \dfrac{1}{2}\left(x + {\beta}^{2}\right)

⇒y=x2β+β2...(i)⇒ y = \dfrac{x}{2\beta} + \dfrac{\beta}{2} . . . \left(i\right)

Tangent to parabola is also a tangent to ellipse

x2+2y2=1⇒x2+y2(12)=1{x}^{2} + 2{y}^{2} = 1 ⇒ {x}^{2} + \dfrac{{y}^{2}}{\left(\dfrac{1}{2}\right)} = 1

And, a line y=mx+cy = mx + c is a tangent to the ellipse x2a2+y2b2=1\dfrac{{x}^{2}}{{a}^{2}} + \dfrac{{y}^{2}}{{b}^{2}} = 1 if c2=a2m2+b2{c}^{2} = {a}^{2}{m}^{2} + {b}^{2}

Using the condition c2=a2m2+b2,{c}^{2} = {a}^{2}{m}^{2} + {b}^{2}, we get

β24=12(12β)2+12\dfrac{{\beta}^{2}}{4} = {1}^{2}{\left(\dfrac{1}{2\beta}\right)}^{2} + \dfrac{1}{2}

⇒β24=14β2+12⇒ \dfrac{{\beta}^{2}}{4} = \dfrac{1}{4{\beta}^{2}} + \dfrac{1}{2}

⇒β4−2β2−1=0⇒ {\beta}^{4} - 2{\beta}^{2} - 1 = 0

Using Sridharacharya's Formula the roots of a quadratic equation ax2+bx+c=0,a≠0a{x}^{2} + bx + c = 0, a \ne 0 are x=−b±b2−4ac2ax = \dfrac{- b \pm \sqrt{{b}^{2} - 4ac}}{2a}

⇒β2=−(−2)±(−2)2−4×1×(−1)2×1⇒ {\beta}^{2} = \dfrac{- \left(- 2\right) \pm \sqrt{{\left(- 2\right)}^{2} - 4 \times 1 \times \left(- 1\right)}}{2 \times 1}

⇒β2=2±82⇒ {\beta}^{2} = \dfrac{2 \pm \sqrt{8}}{2}

⇒β2=1+2⇒ {\beta}^{2} = 1 + \sqrt{2} or β2=1−2({\beta}^{2} = 1 - \sqrt{2} (Neglected as β2{\beta}^{2} is positive))

⇒β2=2+1⇒ {\beta}^{2} = \sqrt{2} + 1

⇒α=2+1.⇒ \alpha = \sqrt{2} + 1 .

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