[数学]已知两个向量，求夹角

// 给出了坐标
//先求出两个向量的模
//再求出两个向量的向量积
//|a|=√[x1^2+y1^2]
//|b|=√[x2^2+y2^2]
//a*b=(x1,y1)(x2,y2)=x1x2+y1y2搜索
//
//cos<a,b>=a*b/[|a|*|b|]
// =(x1x2+y1y2)/[√[x1^2+y1^2]*√[x2^2+y2^2]]

Real a = sqrt((start.x * start.x) + (start.y * start.y));
Real b = sqrt((end.x * end.x) + (end.y * end.y));
Real aXb = (start.x * end.x) + (start.y * end.y);

Real cos_ab = aXb/(a*b);

Real angle_ab = acos(cos_ab)*(180.0/3.141592654);