Question

A parabola has a focus of (-4,1/2) and it’s directrix is at y= 3/2. Write the equation of the function in vertex form

(Show work pls)

Answer 1

Check the picture below.

so the parabola looks more or less like so, since the directrix is above the focus point, the parabola is opening downwards, that means the "p" distance from the focus point or directrix to the vertex is negative.

hmm from the focus point to the directrix is only 1 unit up, and the vertex is half-way between both, so that puts the vertex at (-4 , 1) as you see there, just 1/2 unit above the focus point or 1/2 below the directrix, anyhow

`\textit{vertical parabola vertex form with focus point distance} \\\\ 4p(y- k)=(x- h)^2 \qquad \begin{cases} \stackrel{vertex}{(h,k)}\qquad \stackrel{focus~point}{(h,k+p)}\qquad \stackrel{directrix}{y=k-p}\\\\ p=\textit{distance from vertex to }\\ \qquad \textit{ focus or directrix}\\\\ \stackrel{p~is~negative}{op ens~\cap}\qquad \stackrel{p~is~positive}{op ens~\cup} \end{cases} \\\\[-0.35em] ~\dotfill`

`\begin{cases} h=-4\\ k=1\\ p=-(1)/(2) \end{cases}\implies 4(-(1)/(2))(~~y-1~~) = (~~x-(-4)~~)^2 \\\\\\ -2(y-1)=(x+4)^2\implies y-1=-\cfrac{1}{2}(x+4)^2\implies \boxed{y=-\cfrac{1}{2}(x+4)^2+1}`