##### Synopsis

    #include <math.h>
    double cos(double x);

##### Description

cos returns the radian circular cosine of x.

##### Fast math library behavior

If |x| > 10^9, errno is set to EDOM and cos returns HUGE_VAL.

##### IEC 60559 math library behavior

If x is NaN, cos returns x.
If |x| is ?, cos returns NaN with invalid signal.

##### Portability

cos conforms to ISO/IEC 9899:1990 (C90) and ISO/IEC 9899:1999 (C99).