假设a+b与a-b平行设a(x1,y1),b(x2,y2)根据假设a+b（x1+x2,y1+y2)a-b（x1-x2,y1-y2)（x1+x2)/(y1+y2)=（x1-x2)/(y1-y2)x1y1-x1y2+x2y1-x2y2=x1y1-x2y1+x1y2-x2y2x2y1-x1y2=x1y2-x2y1x2y1=x1y2x1/y1=x2/y2所以a与b平行...