叉积

二维叉积

struct point{
    double x,y;
};
double xmult( point a,point b,point c ){
    return ( b.x-a.x )*( c.y-a.y )-( b.y-a.y )*( c.x-a.x );
}

c

|

|

a-- -- -- -- --b

由b转向a！！

三维叉积

设A（x1,y1,zi)、B（x2,y2,z2)、C(x3,y3,z3)为不共线三点，
向量AB={x2-x1,y2-y1,z2-z1}
向量AC={x3-x1,y3-y1,z3-z1}
由A、B、C三点确定的平面的法向量就是向量AB与向量AC的叉积
三个分量分别是下面的三个二阶行列式
y2-y1        z2-z1                    z2-z1    x2-x1                       x2-x1          y2-y1                          
y3-y1        z3-z1                    z3-z1    x3-x1                       x3-x1          y3-y1

struct point {
    double x,y,z;
};
point func( point a,point b,point c ){
    point one,two,ans;
    one.x=b.x-a.x,one.y=b.y-a.y,one.z=b.z-a.z;
    two.x=c.x-a.x,two.y=c.y-a.y,two.z=c.z-a.z;
    ans.x = one.y*two.z - one.z*two.y;
    ans.y = -one.x*two.z + one.z*two.x;
    ans.z = one.x*two.y - one.y*two.x;
    return ans;
}