三维向量差积，以及应用

Cross u,v gets a new vector uxv

The result of uxv is：

　　uxv=(Yu*Zv–Zu*Yv)*i+(Zu*Xv–Xu*Zv)*j+(Xu*Yv–Yu*Xv)*k

　　We use the determinant to memory it.

Actually，uxv·u=uxv·v=0

　　uxv⊥plane u,v

　　||uxv||=||u||||v||sint (used to calculate the area of △AOB,OA=u,OB=v)

Appling in 2-D plane

　　We get Cross((x1,y1),(x2,y2))=(x1*y2-x2*y1) is the area of a parallelogram