#include <math.h>
double pow (double x, double y);
float powf(float x, float y);
pow() computes x to the power of y (that is, xy).
xy, if x > 0
1, if y == 0
+×, if ( x == 0 && y < 0)
NAN, if ( x < 0 && y is not integral). Also, sets errno to EDOM.
±×, with the same sign as x, if the result is too large.