Radian system of measuring angles
All of the built-in trigonometric functions in FreeBASIC express angles in radians.
Let pi the constant equal to the ratio of the circumference of a circle to its diameter. It can be calculated programmatically by multiplying the arctangent of 1 by 4.
A full circle is divided into 2 * pi radians or 360 degrees, which leads to the following conversions:

    pi = atn(1) * 4
    radians = degrees * pi / 180
    degrees = radians * 180 / pi

These 3 formulas can be expressed by using macros:

    #define   pi          ( atn(1) * 4 )
    #define   radian(x)   ( (x) * pi / 180 )
    #define   degree(x)   ( (x) * 180 / pi )

The added parentheses compared to the raw formulas are mandatory to not undergo an unwanted precedence change of operators when the macros are used in any context of expressions.