a与b夹角 = x
(a-2b).a =0
|a|^2-2a.b =0 (1)
(b-2a).b = 0
|b|^2 -2a.b =0 (2)
=> |a| =|b|
from (1)
cosx =|a|^2/(2|a||b|) = 1/2
x =π/3