A,B,C三点共线<=>存在λ 使得 AB=λAC<=> (x2-x1,y2-y1,z2-z1)=λ(x3-x1,y3-y1,z3-z1)<=> x2-x1=λ(x3-x1),y2-y1=λ(y3-y1),z2-z1=λ(z3-z1).
