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直线的参数表示 P=p+t*v

///直线相交,求两条直线的交点///调用前需保证两直线有唯一的交点，且不重合Vector GetLineIntersection(Point p,Vector v,Point q,Vector w){    Vector u=p-q;    double t=Cross(w,u)/Cross(v,w);    return p+v*t;}///点到直线的距离///用叉积求三角形面积double  DistanceToLine(Point p,Point a,Point b){    Vector v1=b-a;    Vector v2=p-a;    return fabs(Cross(v1,v2))/Length(v1);}///点到线段的最短距离，注意分情况double DistanceToSegment(Point p,Point a,Point b){    if(a==b) return Length(p-a);    Vector v1=b-a,v2=p-a,v3=p-b;    if(dcmp(Dot(v1,v2))<0) return Length(v2);    if(dcmp(Dot(v1,v3))>0) return Length(v3);    return fabs(Cross(v1,v2))/Length(v1);}///求点在直线上的投影点坐标Point GetLine(Point p,Point a,Point b){    Vector v=b-a;    return a+v*(Dot(v,p-a)/Dot(v,v));}///判断线段相交///充要条件：每条线段的端点都在另一条线段的两侧（叉积符号不同）bool SegmentProperIntersection(Point a1,Point a2,Point b1,Point b2){    double c1=Cross(a2-a1,b1-a1),c2=Cross(a2-a1,b2-a1);    double c3=Cross(b2-b1,a1-b1),c4=Cross(b2-b1,a2-b1);    return dcmp(c1)*dcmp(c2)<0&&dcmp(c3)*dcmp(c4)<0;}///判断点是否在线段上bool OnSegment(Point p,Point a1,Point a2){    return dcmp(Cross(a1-p,a2-p))==0&&dcmp(Dot(a1-p,a2-p))<0;}///求多边形面积double PolyArea(Point *p,int n){    double area=0;    for(int i=1; i

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