61.
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
$$\left( { - \frac{p}{2},\, - \frac{p}{2}} \right)$$
The focus of parabola $${y^2} = 2px$$ is $$\left( {\frac{p}{2},\,0} \right)$$ and directrix $$x = - \frac{p}{2}$$
In the figure, we have supposed that $$\left. {p > 0} \right]$$
$$\therefore $$ Centre of circle is $$\left( {\frac{p}{2},\,0} \right)$$ and radius $$ = \frac{p}{2} + \frac{p}{2} = p$$
$$\therefore $$ Equation of circle is $${\left( {x - \frac{p}{2}} \right)^2} + {y^2} = {p^2}$$
For points of intersection of $${y^2} = 2px.....({\text{i}})$$
and $$4{x^2} + 4{y^2} - 4px - 3{p^2} = 0.....({\text{ii}})$$
can be obtained by solving (i) and (ii) as follows
$$\eqalign{
& 4{x^2} + 8px - 4px - 3{p^2} = 0 \cr
& \Rightarrow \left( {2x + 3p} \right)\left( {2x - p} \right) = 0 \cr
& \Rightarrow x = \frac{{ - 3p}}{2},\,\frac{p}{2} \cr
& \Rightarrow y = - 3{p^2}\left( {{\text{not possible}}} \right),\,\,{p^2} \Rightarrow y = \pm p \cr} $$
$$\therefore $$ Required pts are $$\left( {\frac{p}{2},\,p} \right),\,\,\left( {\frac{p}{2},\, - p} \right)$$
62.
Any point on the parabola whose focus is $$\left( {0,\,1} \right)$$ and the directrix is $$x + 2 = 0$$ is given by :
66.
A line $$L$$ passing through the focus of the parabola $${y^2} = 4\left( {x - 1} \right)$$ intersects the parabola in two distinct points. If $$'m’$$ be the slope of the line $$L$$ then :
Let $$y = Y,\,x - 1 = X.$$ Then the equation is $${Y^2} = 4X$$
So, the focus $$ = {\left( {1,\,0} \right)_{X,\,Y}} = \left( {2,\,0} \right)$$
Any line through the focus is $$y = m\left( {x - 2} \right).$$ Solving this with $${y^2} = 4\left( {x - 1} \right),\,{m^2}{\left( {x - 2} \right)^2} = 4\left( {x - 1} \right){\text{ or }}{m^2}{x^2} - 4\left( {{m^2} + 1} \right)x + 4\left( {{m^2} + 1} \right) = 0$$
If $$m \ne 0,\,D = 16{\left( {{m^2} + 1} \right)^2} - 16{m^2}\left( {{m^2} + 1} \right) = 16\left( {{m^2} + 1} \right) > 0{\text{ for all }}m$$
But, if $$m = 0$$ then $$x$$ does not have two real distinct values.
So $$m\, \in \,R$$ except $$m = 0$$
67.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.
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.
68.
If tangents are drawn to the parabola $${y^2} = 4ax$$ at points whose abscissae are in the ratio $${m^2}:1,$$ then the locus of their point of intersection is the curve $$\left( {m > 0} \right).$$
A
$${y^2} = {\left( {{m^{\frac{1}{2}}} - {m^{ - \frac{1}{2}}}} \right)^2}ax$$
B
$${y^2} = {\left( {{m^{\frac{1}{2}}} + {m^{ - \frac{1}{2}}}} \right)^2}ax$$
C
$${y^2} = {\left( {{m^{\frac{1}{2}}} + {m^{ - \frac{1}{2}}}} \right)^2}x$$
Consider two points $$P\left( {at_1^2,\,2a{t_1}} \right)$$ and $$Q\left( {at_2^2,\,2a{t_2}} \right)$$ on the parabola $${y^2} = 4ax$$
Given : $$\frac{{at_1^2}}{{at_2^2}} = \frac{{{m^2}}}{1}{\text{ or }}{t_1} = m{t_2}......\left( 1 \right)$$
Let $$R\left( {h,\,k} \right)$$ be the point of intersection of tangents at $$P$$ and $$Q.$$
Then, $$h = a{t_1}{t_2}$$ and $$k = a\left( {{t_1} + {t_2}} \right)$$
$$\eqalign{
& \Rightarrow h = amt_2^2{\text{ and }}k = a\left( {m{t_2} + {t_2}} \right)\,\,\,\,\,\,\,\,\left[ {{\text{using }}\left( 1 \right)} \right] \cr
& \Rightarrow t_2^2 = \frac{h}{{am}}{\text{ and }}{t_2} = \frac{k}{{a\left( {m + 1} \right)}} \cr} $$
Equating the two values of $${t_2},$$ we get
$$\eqalign{
& \frac{{{k^2}}}{{{a^2}{{\left( {m + 1} \right)}^2}}} = \frac{h}{{am}} \cr
& \Rightarrow {k^2} = ah\frac{{{{\left( {m + 1} \right)}^2}}}{m} \cr
& \Rightarrow {k^2} = ah{\left( {\sqrt m + \frac{1}{{\sqrt m }}} \right)^2} \cr} $$
$$\therefore $$ Required locus is $${y^2} = ax{\left( {{m^{\frac{1}{2}}} + {m^{ - \frac{1}{2}}}} \right)^2}.$$
69.
Tangent to the curve $$y = {x^2} + 6$$ at a point (1, 7) touches the circle $${x^2} + {y^2} + 16x + 12y + c = 0$$ at a point $$Q.$$ Then the coordinates of $$Q$$ are-
The given curve is $$y = {x^2} + 6$$
Equation of tangent at $$\left( {1,\,7} \right)$$ is
$$\eqalign{
& \frac{1}{2}\left( {y + 7} \right) = x.1 + 6 \cr
& \Rightarrow 2x - y + 5 = 0.....(1) \cr} $$
As given this tangent (1) touches the circle
$${x^2} + {y^2} + 16x + 12y + c = 0$$ at $$Q$$
Centre of circle $$ = \left( { - 8,\, - 6} \right).$$
Then equation of $$CQ$$ which is perpendicular to (1) and passes through $$\left( { - 8,\, - 6} \right)$$ is $$y + 6 = - \frac{1}{2}\left( {x + 8} \right)$$
$$ \Rightarrow x + 2y + 20 = 0.....(2)$$
Now $$Q$$ is points of intersection of (1) and (2)
$$\therefore $$ Solving equation (1) & (2) we get
$$x=-6,\,\,y=-7$$
$$\therefore $$ Required points is $$\left( { - 6,\, - 7} \right)$$
70.
Axis of a parabola lies along $$x$$-axis. If its vertex and focus are at distance $$2$$ and $$4$$ respectively from the origin, on the positive $$x$$-axis then which of the following points does not lie on it?
Since, vertex and focus of given parabola is $$\left( {2,\,0} \right)$$ and $$\left( {4,\,0} \right)$$ respectively
Then, equation of parabola is
$$\eqalign{
& {\left( {y - 0} \right)^2} = 4 \times 2\left( {x - 2} \right) \cr
& \Rightarrow {y^2} = 8x - 16 \cr} $$
Hence, the point $$\left( {8,\,6} \right)$$ does not lie on given parabola.