Format: cos( x )
Arguments: (num) x Angle in radians, may be any real or complex number
Returns: (num) The cosine of angle x
Examples: cos(PI) = -1
cos(60deg) = 0.5
cos(1+I) = 0.834-0.989i
See Also: sin, tan, asin, acos, atan, atan2, PI
