Question

The normal to a curve at $$P\left( {x,\,y} \right)$$   meets the $$x$$-axis at $$G.$$  If the distance of $$G$$ from the origin is twice the abscissa of $$P,$$  then the curve is a :

A. circle
B. hyperbola  
C. ellipse
D. parabola
Answer :   hyperbola
Solution :
Equation of normal at $$P\left( {x,\,y} \right)$$   is $$Y - y = - \frac{{dx}}{{dy}}\left( {X - x} \right)$$
Coordinate of $$G$$ at $$X$$ axis is $$\left( {X,\,0} \right)$$  (let)
$$\eqalign{ & \therefore 0 - y = - \frac{{dx}}{{dy}}\left( {X - x} \right) \cr & \Rightarrow y\frac{{dy}}{{dx}} = X - x \cr & \Rightarrow X = x + y\frac{{dy}}{{dx}} \cr} $$
$$\therefore $$ Co-ordinate of $$G\,\,\left( {x + y\frac{{dy}}{{dx}},\,0} \right)$$
Given distance of $$G$$ from origin $$=$$ twice of the abscissa of $$P.$$
$$\because $$ Distance cannot be $$-ve,$$  therefore abscissa $$x$$ should be $$+ve$$
$$\eqalign{ & \therefore x + y\frac{{dy}}{{dx}} = 2x \cr & \Rightarrow y\frac{{dy}}{{dx}} = x \cr & \Rightarrow ydy = xdx \cr} $$
On Integrating $$ \Rightarrow \frac{{{y^2}}}{2} = \frac{{{x^2}}}{2} + {c_1}\,\, \Rightarrow {x^2} - {y^2} = - 2{c_1}$$
$$\therefore $$ The curve is a hyperbola.

Releted MCQ Question on
Geometry >> Hyperbola

Releted Question 1

Each of the four inequalities given below defines a region in the $$xy$$  plane. One of these four regions does not have the following property. For any two points $$\left( {{x_1},\,{y_1}} \right)$$  and $$\left( {{x_2},\,{y_2}} \right)$$  in the the region, the point $$\left( {\frac{{{x_1} + {x_2}}}{2},\,\frac{{{y_1} + {y_2}}}{2}} \right)$$    is also in the region. The inequality defining this region is :

A. $${x^2} + 2{y^2} \leqslant 1$$
B. $${\text{max }}\left\{ {\left| x \right|,\left| y \right|} \right\} \leqslant 1$$
C. $${x^2} - {y^2} \leqslant 1$$
D. $${y^2} - {x^2} \leqslant 0$$
Releted Question 2

Let $$P\left( {a\,\sec \,\theta ,\,b\,\tan \,\theta } \right)$$    and $$Q\left( {a\,\sec \,\phi ,\,b\,\tan \,\phi } \right),$$    where $$\theta + \phi = \frac{\pi }{2},$$   be two points on the hyperbola $$\frac{{{x^2}}}{{{a^2}}} - \frac{{{y^2}}}{{{b^2}}} = 1.$$    If $$\left( {h,\,k} \right)$$  is the point of intersection of the normal at $$P$$ and $$Q,$$  then $$k$$ is equal to :

A. $$\frac{{{a^2} + {b^2}}}{a}$$
B. $$ - \left( {\frac{{{a^2} + {b^2}}}{a}} \right)$$
C. $$\frac{{{a^2} + {b^2}}}{b}$$
D. $$ - \left( {\frac{{{a^2} + {b^2}}}{b}} \right)$$
Releted Question 3

If $$x=9$$  is the chord of contact of the hyperbola $${x^2} - {y^2} = 9,$$   then the equation of the corresponding pair of tangents is :

A. $$9{x^2} - 8{y^2} + 18x - 9 = 0$$
B. $$9{x^2} - 8{y^2} - 18x + 9 = 0$$
C. $$9{x^2} - 8{y^2} - 18x - 9 = 0$$
D. $$9{x^2} - 8{y^2} + 18x + 9 = 0$$
Releted Question 4

For hyperbola $$\frac{{{x^2}}}{{{{\cos }^2}\alpha }} - \frac{{{y^2}}}{{{{\sin }^2}\alpha }} = 1,$$     which of the following remains constant with change in $$'\alpha \,'$$

A. abscissae of vertices
B. abscissae of foci
C. eccentricity
D. directrix

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