If the points $(0,4)$ and $(0,2)$ are respectively the vertex and focus of a parabola, then find the equation of the parabola.
Given that the coordinates, vertex of the parabola $(0,4)$ and focus of the parabola $(0,2)$.
$$ \begin{aligned} &\text { By definition of the parabola, } \quad P B=P F\\ &\begin{array}{cc} \Rightarrow & \left|\frac{0+y-6}{\sqrt{0+1}}\right|=\sqrt{(x-0)^2+(y-2)^2} \\ \Rightarrow & |y-6|=\sqrt{x^2+y^2-4 y+4} \\ \Rightarrow & x^2+y^2-4 y+4=y^2+36-12 y \\ \Rightarrow & x^2+8 y=32 \end{array} \end{aligned}$$
If the line $y=m x+1$ is tangent to the parabola $y^2=4 x$, then find the value of $m$.
Given that, line $y=m x+1$ is tangent to the parabola $y^2=4 x$.
$\therefore\quad y =m x+1\quad\text{... (i)}$
$\text{and}\quad y^2 =4 x\quad \text{... (ii)}$
From Eqs. (i) and (ii),
$$\begin{array}{r} & m^2 x^2+2 m x+1 =4 x \\ \Rightarrow & m^2 x^2+2 m x-4 x+1 =0 \\ \Rightarrow & m^2 x^2+x(2 m-4)+1 =0 \\ \Rightarrow & (2 m-4)^2-4 m^2 \times 1 =0 \\ \Rightarrow & 4 m^2+16-16 m-4 m^2 =0 \\ \Rightarrow & 16 m =16 \\ \therefore & m=1 \end{array}$$
If the distance between the foci of a hyperbola is 16 and its eccentricity is $\sqrt{2}$, then obtain the equation of the hyperbola.
$$\begin{aligned} \text{Distance between the foci i.e.,}\quad 2 a e =16 \Rightarrow a e=8 \\ \text{and}\quad e =\sqrt{2} \\ \therefore\quad a \sqrt{2} =8 \\ a =4 \sqrt{2} \\ \text{We know that,}\quad b^2 =a^2\left(e^2-1\right) \\ b^2 =(4 \sqrt{2})^2\left[(\sqrt{2})^2-1\right] \\ =16 \times 2(2-1) \\ =32(2-1) \end{aligned}$$
So, the equation of hyperbola is
$$\begin{aligned} & \frac{x^2}{32}-\frac{y^2}{32}=1 \\ & \Rightarrow \quad x^2-y^2=32 \end{aligned}$$
21 Find the eccentricity of the hyperbola $9 y^2-4 x^2=36$.
Given equation of the hyperbola is
$$\begin{array}{ll} & 9 y^2-4 x^2=36 \\ \Rightarrow & \frac{9 y^2}{36}-\frac{4 x^2}{36}=\frac{36}{36} \\ \Rightarrow & \frac{y^2}{4}-\frac{x^2}{9}=1 \\ \Rightarrow & -\frac{x^2}{9}+\frac{y^2}{4}=1 \end{array}$$
Since, this equation in form of $-\frac{x^2}{a}+\frac{y^2}{b^2}=1$, where $a=3$ and $b=2$.
$$\begin{aligned} \therefore \quad e & =\sqrt{1+\left(\frac{a}{b}\right)^2} \\ & =\sqrt{1+\frac{9}{4}}=\frac{\sqrt{13}}{2} \end{aligned}$$
Find the equation of the hyperbola with eccentricity $\frac{3}{2}$ and foci at $( \pm 2,0)$.
Given that eccentricity i.e., $e=3 / 2$ and $( \pm a e, 0)=( \pm 2,0)$
$$\begin{array}{ll} \therefore & a e=2 \\ \Rightarrow & a \cdot \frac{3}{2}=2 \Rightarrow a=4 / 3 \\ \because & b^2=a^2\left(e^2-1\right) \\ \Rightarrow & b^2=\frac{16}{9}\left(\frac{9}{4}-1\right) \\ \Rightarrow & b^2=\frac{16}{4}\left(\frac{5}{4}\right)=+\frac{20}{9} \end{array}$$
So, the equation of hyperbola is
$$\begin{aligned} & \frac{x^2}{\frac{16}{9}}-\frac{y^2}{\frac{20}{9}}=1 \\ & \Rightarrow \quad \frac{x^2}{4}-\frac{y^2}{5}=\frac{4}{9} \end{aligned}$$