#include <cmath> double asin( double arg );
The asin() function returns the arc sine of arg, which will be in the range [- pi/2, +pi/2]. arg should be between -1 and 1. If arg is outside this range, asin() returns NAN and raises a floating-point exception.
C++ also provides the following overloaded forms:
#include <cmath> float asin( float arg ); // same as asinf() in C99 long double asin( long double arg ); // same as asinl() in C99