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
Vertex $=(0,4)$ Focus $=(0,2)$
So, the directrix of the parabola is $y=6$,
Since, distance of $(x, y)$ from $(0,2)$ and perpendicular distance from $(x, y)$ to directrix are always equal.
Using Distance Formula \& Perpendicular Distance Formula, Perpendicular Distance (Between a point and line) is
$=\frac{\left|a x_{1}+b y_{1}+c\right|}{\sqrt{a^{2}+b^{2}}}$
Where the point is $\left(x_{1}, y_{1}\right)$ and the line is expressed as ax $+$ by $+c=0$ i.e.., $x(0)+y-6=0 \&(x, y)$
Distance between the point of intersection \& centre
We know distance formula $=\sqrt{\left(x_{2}-x_{1}\right)^{2}+\left(y_{2}-y_{1}\right)^{2}}$
Now by substituting the values we get
$\sqrt{\left((x-0)^{2}+(y-2)^{2}\right)}=\frac{|x(0)+y(1)-6|}{\sqrt{0^{2}+\left(-1^{2}\right)}}=\frac{y-6}{1}=y-6$
Squaring both the sides,
$\left[\sqrt{\left((x-0)^{2}+(y-2)^{2}\right)}\right]^{2}=(y-6)^{2}$
$x^{2}+y^{2}-4 y+4=y^{2}-12 y+36$
$x^{2}+8 y-32=0$
Hence, the required equation is $x^{2}+8 y-32=0$.