Question

Statement-1 : An equation of a common tangent to the parabola $${y^2} = 16\sqrt 3 x$$   and the ellipse $$2{x^2} + {y^2} = 4$$    is $$y = 2x + 2\sqrt 3 $$
Statement-2 : If the line $$y = mx + \frac{{4\sqrt 3 }}{m},\,\left( {m \ne 0} \right)$$     is a common tangent to the parabola $${y^2} = 16\sqrt 3 x$$   and the ellipse $$2{x^2} + {y^2} = 4,$$    then $$m$$ satisfies $${m^4} + 2{m^2} = 24$$

A. Statement-1 is false, Statement-2 is true.
B. Statement-1 is true, statement-2 is true; statement-2 is a correct explanation for Statement-1.  
C. Statement-1 is true, statement-2 is true; statement-2 is not a correct explanation for Statement-1.
D. Statement-1 is true, statement-2 is false.
Answer :   Statement-1 is true, statement-2 is true; statement-2 is a correct explanation for Statement-1.
Solution :
Given equation of ellipse is $$2{x^2} + {y^2} = 4$$
$$ \Rightarrow \frac{{2{x^2}}}{4} + \frac{{{y^2}}}{4} = 1\,\,\,\,\, \Rightarrow \frac{{{x^2}}}{2} + \frac{{{y^2}}}{4} = 1$$
Equation of tangent to the ellipse $$\frac{{{x^2}}}{2} + \frac{{{y^2}}}{4} = 1$$    is
$$y = mx \pm \sqrt {2{m^2} + 4} .....(1)$$
($$\because $$ equation of tangent to the ellipse $$\frac{{{x^2}}}{{{a^2}}} + \frac{{{y^2}}}{{{b^2}}} = 1$$    is $$y=mx+c$$   where $$c = \pm \sqrt {{a^2}{m^2} + {b^2}} $$    )
Now, Equation of tangent to the parabola
$${y^2} = 16\sqrt 3 x$$   is $$y = mx + \frac{{4\sqrt 3 }}{m}.....(2)$$
($$\because $$ equation of tangent to the parabola $${y^2} = 4ax$$   is $$y = mx + \frac{a}{m}$$   )
On comparing (1) and (2), we get
$$\frac{{4\sqrt 3 }}{m} = \pm \sqrt {2{m^2} + 4} $$
Squaring on both the sides, we get
$$\eqalign{ & 16\left( 3 \right) = \left( {2{m^2} + 4} \right){m^2} \cr & \Rightarrow 48 = {m^2}\left( {2{m^2} + 4} \right) \cr & \Rightarrow 2{m^4} + 4{m^2} - 48 = 0 \cr & \Rightarrow {m^4} + 2{m^2} - 24 = 0 \cr & \Rightarrow \left( {{m^2} + 6} \right)\left( {{m^2} - 4} \right) = 0 \cr & \Rightarrow {m^2} = 4\left( {\because {m^2} \ne - 6} \right) \cr & \Rightarrow m = \pm 2 \cr} $$
$$ \Rightarrow $$ Equation of common tangents are $$y = \pm 2x \pm 2\sqrt 3 $$
Thus, statement-1 is true.
Statement-2 is obviously true.

Releted MCQ Question on
Geometry >> Parabola

Releted Question 1

Consider a circle with its centre lying on the focus of the parabola $${y^2} = 2px$$   such that it touches the directrix of the parabola. Then a point of intersection of the circle and parabola is-

A. $$\left( {\frac{p}{2},\,p} \right){\text{ or }}\left( {\frac{p}{2},\, - p} \right)$$
B. $$\left( {\frac{p}{2},\, - \frac{p}{2}} \right)$$
C. $$\left( { - \frac{p}{2},\,p} \right)$$
D. $$\left( { - \frac{p}{2},\, - \frac{p}{2}} \right)$$
Releted Question 2

The curve described parametrically by $$x = {t^2} + t + 1,\,\,y = {t^2} - t + 1$$      represents-

A. a pair of straight lines
B. an ellipse
C. a parabola
D. a hyperbola
Releted Question 3

If $$x+y=k$$   is normal to $${y^2} = 12x,$$   then $$k$$ is-

A. $$3$$
B. $$9$$
C. $$ - 9$$
D. $$ - 3$$
Releted Question 4

If the line $$x-1=0$$   is the directrix of the parabola $${y^2} - kx + 8 = 0,$$    then one of the values of $$k$$ is-

A. $$\frac{1}{8}$$
B. $$8$$
C. $$4$$
D. $$\frac{1}{4}$$

Practice More Releted MCQ Question on
Parabola


Practice More MCQ Question on Maths Section