Question 1121083
{{{y=ax^2}}}

{{{(1/a)y=x^2}}}
Imagine the parabola cross section with vertex at (0,0) and concave up.  A point is  ( 9.375, 3.66 ).


{{{4d=1/a}}}

and
{{{(1/a)*3.66=9.375^2}}}

{{{1/a=(9.375^2)/3.66}}}

{{{1/a=24.01383197=4d}}}

{{{d=6.0035}}}

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Place the microphone at focus 6.0 inches above the vertex.