向量可以直接上坐标设a(1,0),b(x,y)，则a-b=(1-x,-y)|a-b|=|a|，所以(x-1)^2+y^2 = 1a(a-b) = 1-x = 0所以x=1.y=1a-b = (0,-1)和b的夹角就是135°
