ATAN()
 Computes the tangent arc
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 Syntax

     ATAN(<nTangent>) --> nArcTan

 Argument

     <nTangent>  Designates the angle tangent value for which you want
     the response determined in radians.

 Returns

     ATAN() returns the tangent arc for <nTangent>.

 Description

     ATAN() determines an angle size in radians for a tangent value.

 Note

     .  The SETPREC() function can influence the precision of the
        result.

 Example

     Compute the arc of the tangent :

     ? STR(ATAN(PI()/4), 18, 15)      // 0.665773750028354