Question 1037089
Cross section is a parabola shape.  Imagine the vertex at the Origin, and this vertex as a minimum.


Radius is half of the diameter, being radius of 200.  "Depth" becomes there distance of 80 meters.  This is a cartesian point,  (200,80).  Standard Form can start as  {{{y=a(x-0)^2+0}}},  the two 0 values because the vertex is (0,0), the Origin as picked here.  This more simply is {{{y=ax^2}}}.


Use the point on the rim of the cross section to find coefficient, a.
{{{a=y/x^2}}}
{{{a=80/200^2}}}
{{{a=80/40000}}}
{{{a=2/1000}}}
{{{a=1/500}}}
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Finished equation for the cross section is  {{{highlight(y=(1/500)x^2)}}}.


You can find the focus using this video presentation's information as help:
<a href="https://www.youtube.com/watch?v=M8LGsQMwwj4">Deriving Equation for Parabola using given Focus and Directrix</a>