ATN returns the arctangent of a specified numeric expression.
COS, SIN, and TAN return the cosine, sine, and tangent of a specified angle.
 
ATN(numeric-expression)
COS(angle)
SIN(angle)
TAN(angle)
 
    - numeric-expression    The ratio between the sides of a right triangle.
    - angle                 An angle expressed in radians.
 
    - The ATN function returns an angle in radians.
    - To convert from degrees to radians, multiply degrees by (PI / 180).
 
Example:
    CONST PI=3.141592654
    PRINT ATN(TAN(PI/4.0)), PI/4.0    'Output is:  .7853981635  .7853981635
    PRINT (COS(180 * (PI / 180)))     'Output is:  -1
    PRINT (SIN(90 * (PI / 180)))      'Output is:  1
    PRINT (TAN(45 * (PI / 180)))      'Output is:  1.000000000205103