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 at a point \left(\alpha, \beta\right), \left(\beta > 0\right) is also a tangenttotheellipse,x2+2y2=1 then α is equal to:
Options
A2−1
B22+1
C2+1
D22−1
Correct answer
C. 2+1
Step-by-step solution
Since (α,β) lies on the parabola ⇒α=β2
⇒(α,β)=(β2,β)
Tangent to a parabola y2=4ax at a point \left({x}_{1}, {y}_{1}\right)isy{y}_{1} = 2a\left(x + {x}_{1}\right)</p><p>Therefore,tangenttotheparabola{y}^{2} = xat(β2,β) isy⋅β=21(x+β2)
⇒y=2βx+2β...(i)
Tangent to parabola is also a tangent to ellipse
x2+2y2=1⇒x2+(21)y2=1
And, a line y=mx+c is a tangent to the ellipse a2x2+b2y2=1 if c2=a2m2+b2
Using the condition c2=a2m2+b2, we get
4β2=12(2β1)2+21
⇒4β2=4β21+21
⇒β4−2β2−1=0
Using Sridharacharya's Formula the roots of a quadratic equation ax2+bx+c=0,a=0 are x=2a−b±b2−4ac