求向量夹角

向量夹角公式：

　　cosα = A*B / (|A|*|B|)

　　设A(x1,y1),B(x2,y2);

　　cosα =(x1*x2 + y1*y2) / sqrt( (x1^2 + y1^2) * (x2^2 + y2^2) )

代码实现(c++)：

#include <math.h> 

//Calcucate angle between vector A and B
//返回角度; return degree
int CalculateVectorAngle(int x1, int y1, int x2, int y2)
{  
  //acos return radian,we should transform it into degree  
  return acos((x1*x2 + y1*y2) / sqrt((x1*x1 + y1*y1)*(x2*x2 + y2*y2)) * 180 / 3.14;
}