设单位向量b=(x,y)与a垂直,则有
x²+y²=1
4x+2y=0
解得x=根号5/5,y=-2根号5/5或x=-根号5/5,y=2根号5/5
与a垂直的单位向量的坐标有两个
(根号5/5,-2根号5/5)或(-根号5/5,2根号5/5)