#include <math.h>float atan2f(float a, float b);double atan2(double a, double b); long double atan2l(long double a, long double b);
atan2f( ) and atan2l( ) were added by C99.
The atan2( ) family of functions returns the arc tangent of a/b. It uses the signs of its arguments to compute the quadrant of the return value.
Related functions are asin( ), acos( ), atan( ), tan( ), cos( ), sin( ), sinh( ), cosh( ), and tanh( ).