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Question: Answered & Verified by Expert
If the tangent to the parabola y2=x at a point α,β,β>0 is also a tangent to the ellipse, x2+2y2=1 then α is equal to:
MathematicsEllipseJEE MainJEE Main 2019 (09 Apr Shift 2)
Options:
  • A 2-1
  • B 22+1
  • C 2+1
  • D 22-1
Solution:
2119 Upvotes Verified Answer
The correct answer is: 2+1
Since α, β lies on the parabola α=β2

α, β=β2, β

Tangent to a parabola y2=4ax at a point x1, y1 is yy1=2ax+x1

Therefore, tangent to the parabola y2=x at β2, β isyβ=12x+β2

y=x2β+β2   ...i

Tangent to parabola is also a tangent to ellipse

x2+2y2=1 x2+y212=1

And, a line y=mx+c is a tangent to the ellipse x2a2+y2b2=1 if c2=a2m2+b2

Using the condition c2=a2m2+b2, we get

β24=1212β2+12

β24=14β2+12

β4-2β2-1=0

Using Sridharacharya's Formula the roots of a quadratic equation ax2+bx+c=0, a0 are x=-b±b2-4ac2a

β2=--2±-22-4×1×-12×1

β2=2±82

β2=1+2 or β2=1-2 (Neglected as β2 is positive)

β2=2+1

α=2+1.

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