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