(a-2b)⊥a,则(a-2b)*a=0,则a^2=2ab=|a|^2,则|a|=sqr(2ab)
(b-2a)⊥b,则(b-2a)*b=0,则b^2=2ab,|b|=sqr(2ab)
cos=a*b/(|a|*|b|)=1/2
=兀/3
B