Given parabola $${x^2} = 12y$$ is which is of the form $${x^2} = 4ay$$
$$ \Rightarrow 4a = 12 \Rightarrow a = 3$$
Now, $$LM$$ is the latus rectum whose length $$ = 4a = 4 \times 3 = 12$$
So, area of $$\Delta LMV$$
$$\eqalign{
& = \frac{1}{2} \times LM \times VF \cr
& = \left( {\frac{1}{2} \times 12 \times 3} \right){\text{square units}} \cr
& = 18{\text{ square units}} \cr} $$
42.
The range of values of $$\lambda $$ for which the point $$\left( {\lambda ,\, - 1} \right)$$ is exterior to both the parabolas $${y^2} = \left| x \right|$$ is :
The parabolas are $${y^2} - x = 0$$ and $${y^2} + x = 0.$$ The point $$\left( {\lambda ,\, - 1} \right)$$ is an exterior point if $$1 - \lambda > 0$$ and $$1 + \lambda > 0\,\,\, \Rightarrow \lambda < 1$$ and $$\lambda > - 1\,\,\, \Rightarrow - 1 < \lambda < 1.$$
43.
A parabola has the origin as its focus and the line $$x = 2$$ as the directrix. Then the vertex of the parabola is at-
Vertex of a parabola is the mid point of focus and the point where directrix meets the axis of the parabola.
Here focus is $$O\left( {0,\,0} \right)$$ and directrix meets the axis at $$B\left( {2,\,0} \right)$$
$$\therefore $$ Vertex of the parabola is $$\left( {1,\,0} \right).$$
44.
If $$x+y=k$$ is normal to $${y^2} = 12x,$$ then $$k$$ is-
$$y=mx+c$$ is normal to the parabola
$${y^2} = 4ax$$ if $$c = - 2am - a{m^3}$$
Here $$m=-1, \,c=k$$ and $$a=3$$
$$\therefore c = k = - 2\left( 3 \right)\left( { - 1} \right) - 3{\left( { - 1} \right)^3} = 9$$
45.
The number of points with integral coordinates that lie in the interior of the region common to the circle $${x^2} + {y^2} = 16$$ and the parabola $${y^2} = 4x$$ is :
$$\left( {\lambda ,\,\mu } \right)$$ is interior to both the curves if $${\lambda ^2} + {\mu ^2} - 16 < 0$$ and $${\mu ^2} - 4\lambda < 0$$
Now, $${\mu ^2} - 4\lambda < 0\,\,\,\, \Rightarrow \lambda > {\left( {\frac{\mu }{2}} \right)^2}$$
Hence, if $$\mu = 0,\,\lambda = 1,\,2,\,3,\,.....;$$ if $$\mu = 1,\,\lambda = 1,\,2,\,3,\,.....;$$ if $$\mu = 2,\,\lambda = 2,\,3,\,.....;$$ if $$\mu = 3,\,\lambda = 3,\,4\,.....;$$
Also $${\lambda ^2} + {m^2} - 16 < 0\,\,\,\,\, \Rightarrow {\lambda ^2} < 16 - {\mu ^2}$$
Hence, if $$\mu = 0,\,\lambda = 1,\,2,\,3\,;$$ if $$\mu = 1,\,\lambda = 1,\,2,\,3\,;$$ if $$\mu = 2,\,\lambda = 2,\,3\,;$$ if $$\mu = 3,\,\lambda $$ has no integral value.
$$\therefore \,\,\left( {1,\,0} \right),\,\left( {2,\,0} \right),\,\left( {3,\,0} \right),\,\left( {1,\,1} \right),\,\left( {2,\,1} \right),\,\left( {3,\,1} \right),\,\left( {2,\,2} \right),\,\left( {3,\,2} \right)$$ are the possible points.
46.
Let $$O$$ be the vertex and $$Q$$ be any point on the parabola, $${x^2} = 8y.$$ If the point $$P$$ divides the line segment $$OQ$$ internally in the ratio $$1 : 3,$$ then locus of $$P$$ is :
Let $$P\left( {h,\,k} \right)$$ divides $$OQ$$ in the ratio $$1 : 3$$
Let any point $$Q$$ on $${x^2} = 8y$$ is $$\left( {4t,\,2{t^2}} \right).$$
Then by section formula
$$\eqalign{
& \Rightarrow k = \frac{{{t^2}}}{2}{\text{ and }}h = t \cr
& \Rightarrow 2k = {h^2} \cr} $$
Required locus of $$P$$ is $${x^2} = 2y$$
47.
The equation of the common tangent to the curves $${y^2} = 8x$$ and $$xy=-1$$ is-
The given curves are
$$\eqalign{
& \,\,\,\,\,\,\,\,\,\,{y^2} = 8x.....(1) \cr
& {\text{and }}xy = - 1.....(2) \cr} $$
If $$m$$ is the slope of tangent to (1), then equation of tangent is
$$y = mx + \frac{2}{m}$$
If this tangent is also a tangent to (2), then putting value of $$y$$ in curve (2)
$$\eqalign{
& x = \left( {mx + \frac{2}{m}} \right) = - 1 \cr
& \Rightarrow m{x^2} + \frac{2}{m}x + 1 = 0 \cr
& \Rightarrow {m^2}{x^2} + 2x + m = 0 \cr} $$
We should get repeated roots for the equation (condition of tangency)
$$\eqalign{
& \Rightarrow D = 0 \cr
& \therefore {\left( 2 \right)^2} - 4{m^2}.m = 0 \cr
& \Rightarrow {m^3} = 1\,\,\,\,\, \Rightarrow m = 1 \cr} $$
Hence required tangent is $$y=x +2$$
48.
The triangle formed by the tangents to a parabola $${y^2} = 4ax$$ at the ends of the latus rectum and the double ordinate through the focus is :
A
equilateral
B
isosceles
C
right-angled isosceles
D
dependent on the value of $$a$$ for its classification
The tangents to the parabola $${y^2} = 4ax$$ at the points $$\left( {a,\,2a} \right),\,\left( {a,\, - 2a} \right)$$ are $$y = x + a$$ and $$y = - x - a.$$ The third side of the triangle is $$x = a.$$ Clearly, these lines form a right-angled triangle whose two sides are equal.
49.
Equation of the parabola whose vertex is $$\left( { - 3,\, - 2} \right),$$ axis is horizontal and which passes through the point $$\left( {1,\,2} \right)$$ is :
Since the axis is horizontal and vertex is $$\left( { - 3,\, - 2} \right),$$
$$\therefore $$ the equation of the parabola must be of the form $${\left( {y + 2} \right)^2} = 4a\left( {x + 3} \right)$$
It passes through $$\left( {1,\,2} \right),$$ so $$16 = 16a$$ i.e $$a = 1.$$
Hence, the equation of the required parabola is
$${\left( {y + 2} \right)^2} = 4\left( {x + 3} \right){\text{ or }}{y^2} + 4y - 4x - 8 = 0$$
50.
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-