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///点积double Dot(Vector v1,Vector v2){ return v1.x*v2.x+v1.y*v2.y;}double Length(Vector v){ return sqrt(Dot(v,v));}double Angle(Vector v1,Vector v2){ return acos(Dot(v1,v2)/Length(v1)/Length(v2));}///叉积double Cross(Vector v1,Vector v2){ return v1.x*v2.y-v1.y*v2.x;}double Area(Vector v1,Vector v2,Vector v3){ return Cross(v1-v2,v3-v2);}///旋转Vector Rotate(Vector v1,double a) ///可用正余弦函数得出{ return Vector(v1.x*cos(a)-v1.y*sin(a),v1.x*sin(a)+v1.y*cos(a));}///求单位法向量Vector Normal(Vector v){ double L=Length(v); return Vector(-v.y/L,v.x/L);}