71.
The ends of a line segment are $$P\left( {1,\,3} \right)$$ and $$Q\left( {1,\,1} \right).\,R$$ is a point on the line segment $$PQ$$ such that $$PR:QR = 1:\lambda .$$ If $$R$$ is an interior point of the
parabola $${y^2} = 4x$$ then :
A
$$\lambda \, \in \,\left( {0,\,1} \right)$$
B
$$\lambda \, \in \,\left( { - \frac{3}{5},\,1} \right)$$
C
$$\lambda \, \in \,\left( {\frac{1}{2},\,\frac{3}{5}} \right)$$
$${y^2} - 2y + 1 = x - 1{\text{ or }}{\left( {y - 1} \right)^2} = x - 1$$
Putting $$x - 1 = X,\,\,y - 1 = Y$$ the equation becomes $${Y^2} = X,$$ i.e., $${Y^2} = 4.\frac{1}{4}.X.$$ So, the focus $$ = {\left( {\frac{1}{4},\,0} \right)_{X,\,Y}} = \left( {\frac{5}{4},\,1} \right).$$
74.
Let $$P$$ be the point on the parabola, $${y^2} = 8x$$ which is at a minimum distance from the centre $$C$$ of the circle, $${x^2} + {\left( {y + 6} \right)^2} = 1.$$ Then the equation of the circle, passing through $$C$$ and having its centre at $$P$$ is:
Here, $${y^2} - 2y + 1 = 4\left( {x + 1} \right){\text{ or }}{\left( {y - 1} \right)^2} = 4\left( {x + 1} \right)$$
So, the axis is $$y - 1 = 0.$$ Also $$\left( { - 2,\,1} \right)$$ lies on the axis, and it is exterior to the parabola because $${1^2} - 4\left( { - 2} \right) - 2\left( 1 \right) - 3 > 0.$$ Hence, only one normal is possible.
76.
A tangent to the parabola $${y^2} = 8x,$$ which makes an angle of $${45^ \circ }$$ with the straight line $$y = 3x + 5$$ is :
Any tangent to $${y^2} = 4ax$$ is $$y = mx + \frac{a}{m}.$$ It touches $${x^2} = 4ay$$ if $${x^2} = 4a\left( {mx + \frac{a}{m}} \right),$$ i.e., $${x^2} - 4amx - \frac{{4{a^2}}}{m} = 0$$ has equal roots.
So, $${m^3} + 1 = 0,$$ i.e., $$m = - 1.$$ Hence, the common tangent is $$y = - x - a.$$
78.
If the tangent at $$\left( {1,\,7} \right)$$ to the curve $${x^2} = y - 6$$ touches the circle $${x^2} + {y^2} + 16x + 12y + c = 0$$ then the value of $$c$$ is :
Equation of tangent at $$\left( {1,\,7} \right)$$ to $${x^2} = y - 6$$ is $$2x - y + 5 = 0.$$
Now, perpendicular from centre $$O\left( { - 8,\, - 6} \right)$$ to $$2x-y+5=0$$ should be equal to radius of the circle
$$\eqalign{
& \therefore \left| {\frac{{ - 16 + 6 + 5}}{{\sqrt 5 }}} \right| = \sqrt {64 + 36 - C} \cr
& \Rightarrow \sqrt 5 = \sqrt {100 - c} \cr
& \Rightarrow c = 95 \cr} $$
79.
The vertex of the parabola $${y^2} = 8x$$ is at the centre of a circle and the parabola cuts the circle at the ends of its latus rectum. Then the equation of the circle is :