To "freeze" cell references you have to make them absolute using the $

If you want to keep the column index the same put a $ in front of the
column letter and likewise if you want to freeze a row. So if you want
the reference to I3 to remain in tact when copying down the formula, you
would change it to $I$3


So now your formula becomes:
=ATAN(2*B3*$I$4/(2-(B3*$I$3)))


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Vito
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