设A(Xa,Xa);b(Xb,yb)
则向量a+b:(Xa+Xb,Ya+Yb);向量a-b:(Xa-Xb,Ya-Yb)
向量a+b与a-b为平行向量时:(Xa+Xb)(Ya-yB)=(Xa-Xb)(Ya+yB)==#XaYb=XbYa==#向量A//B时
另为零向量时.