#include <math.h>float cosf(float arg);double cos(double arg); long double cosl(long double arg);
cosf( ) and cosl( ) were added by C99.
The cos( ) family of functions returns the cosine of arg. The value of arg must be in radians.
Related functions are asin( ), acos( ), atan2( ), atan( ), tan( ), sin( ), sinh( ), cosh( ), and tanh( ).