|a+b|=|a-b| |a+b|^2=|a-b|^2 a*b=0 ∴ a垂直b |a-2b｜=√（｜a-2b|^2=√6 cos@=(a-2b)*b/|b||a-2b|=-√6/3 @=arccos(-√6/3)